MOSCOW/Ankara - Russia has deployed its advanced S-400 air defence system in Syria, the Russian defence ministry said Thursday, with the weapons to be used to cover the area around its airbase in coastal Latakia.

“According to the decision of the supreme commander-in-chief, an air defence complex S-400 has been delivered to the Russian Hmeimim airbase in Syria and is already on duty to cover the area,” the defence ministry said in a statement.

A video posted on the ministry’s official Facebook page showed launchers being unloaded from a cargo plane, driven down a road, and switched into a deployed position. After Turkey shot down one of its jets, Moscow said Wednesday it would deploy the system in Syria to protect its planes, which have been carrying out air strikes since September 30.

The incident, which Turkey says was caused by the Russian pilot’s breach of its air space, has severely hit relations between the two countries, with President Vladimir Putin calling it a “stab in the back.”

US officials have criticised the decision to send the missiles to Syria, saying their presence raises “significant concerns” due to their wide 400-kilometre (250-mile) reach that stretches beyond Syria’s borders into Turkey.

Defence ministry spokesman Igor Konashenkov said Thursday that the military had been “sure that Russian planes destroying IS targets are guaranteed not to be attacked from the US-led so-called anti-IS coalition.”

But the downing of the Russian jet on Tuesday changed this and “now the safety of Russian fleet’s planes will be ensured by more secure means,” he said.

Turkey summoned the Russian ambassador to protest “physical attacks” on its missions and companies operating in Russia, the foreign ministry said in a statement on Thursday. Turkish missions and firms have been singled out for attacks under the auspices of protests, the foreign ministry said. Moscow has threatened economic retaliation against Turkey after a Russian warplane was downed near the Syrian border this week.

Russia threatened economic retaliation against Turkey on Thursday and said it was still awaiting a reasonable explanation for the shooting down of its warplane, but Turkey dismissed the threats as “emotional” and “unfitting”.

In an escalating war of words, President Tayyip Erdogan responded to Russian accusations that Turkey has been buying oil and gas from Islamic State in Syria by accusing Syrian President Bashar al-Assad and his backers, which include Moscow, of being the real source of the group’s financial and military power.

The shooting down of the jet by the Turkish air force on Tuesday was one of the most serious clashes between a Nato member and Russia, and further complicated international efforts to battle Islamic State militants. World leaders have urged both sides to avoid escalation.

Russian Prime Minister Dmitry Medvedev on Thursday ordered his government to draw up measures that would include freezing some joint investment projects and restricting food imports from Turkey.

Economy Minister Alexei Ulyukayev said Moscow could put limits on flights to and from Turkey, halt preparations for a joint free trade zone, and restrict high-profile projects including the TurkStream gas pipeline and a $20 billion nuclear power plant Russia is building in Turkey.

“We are strategic partners ... ‘Joint projects may be halted, ties could be cut’? Are such approaches fitting for politicians?,” Erdogan said in a speech in Ankara.

“First the politicians and our militaries should sit down and talk about where errors were made and then focus on overcoming those errors on both sides. But instead, if we make emotional statements like this, that wouldn’t be right.”

Kremlin spokesman Dmitry Peskov said Russia was still awaiting a reasonable answer from Ankara on why it downed the fighter jet. Moscow insists it never left Syrian air space, but Ankara says it crossed the border despite repeated warnings.

Erdogan said the Russian jet was shot down as an “automatic reaction” to the violation of Turkish air space, in line with standing orders given to the military.

Those instructions were a separate issue to disagreements with Russia over Syria policy, he said, adding Ankara would continue to support moderate rebels in Syria and Turkmen fighters battling President Assad’s forces. He told CNN that Russia, not Turkey, should be the one to apologise for the incident.

Prime Minister David Cameron on Thursday argued his case to MPs for Britain to join air strikes in Syria ahead of a vote expected at a later date, with signs of opposition weakening after the Paris attacks.

“If we won’t act now, when our friend and ally France has been struck in this way, then our friends and allies can be forgiven for asking: If not now, when?” Cameron asked parliament.

Cameron argued there was a legal basis for intervention for self-defence because of the threat posed by Islamic State militants at home, and said Britain should not “sub-contract” its security to allies.

“We have to deny a safe haven for ISIL in Syria. The longer ISIL is allowed to grow in Syria, the greater the threat it will pose,” he said in a written statement on the issue, using another word for IS.

Saudi foreign minister Adel al-Jubeir said on Thursday that a military option in Syria was still viable and support for the opposition fighting to oust President Bashar al-Assad would continue.

Speaking at a news conference with visiting Austrian Foreign Minister Sebastian Kurz, Jubeir also said Riyadh was in contact with various Syrian opposition groups about a potential meeting in the kingdom to unify their position ahead of upcoming peace talks in Vienna. Jubeir did not give a date for any meeting.

French President Francois Hollande will hold talks on Thursday with Russian leader Vladimir Putin as part of his diplomatic marathon to forge a broad coalition against Islamic State militants in the wake of the Paris attacks.

The French president met Italian Prime Minister Matteo Renzi in Paris early Thursday and will later head to Moscow. Hollande has been on a whirlwind tour seeking to build a coalition to crush IS in Iraq and Syria but has won few concrete pledges so far, and his campaign has been further complicated by a spat between Russia and Turkey over a downed jet.

Medvedev on Wednesday alleged that Turkish officials were benefiting from Islamic State oil sales, while Russian Foreign Minister Sergei Lavrov said it was no secret that “terrorists” use Turkish territory.

“Shame on you. It’s clear where Turkey buys its oil and gas ... Those who claim we are buying oil from Daesh like this must prove their claims. Nobody can slander this country,” Erdogan said, using an Arabic acronym for Islamic State.

“If you are seeking the source of weaponry and financial power of Daesh, the first place to look is the Assad regime and countries that act with it,” he said.

Moscow says its military involvement in Syria is aimed at battling terrorist groups including Islamic State, casting the campaign to a supportive Russian public as a moral crusade that must be completed despite obstruction from elsewhere.

Turkey and its allies say Russia’s real aim is to prop up its ally Assad and that it has been bombing moderate opposition groups in areas of Syria like Latakia, where the jet was downed, and where there is little or no Islamic State presence.

Russian forces have shown no sign of backing down, launching a heavy bombardment against insurgent-held areas in Latakia on Wednesday, near where the jet crashed.

A Reuters correspondent on the Turkish side of the border saw rockets and tank shells being fired from government-controlled western Latakia eastwards into rebel-held territory, sending plumes of smoke rising from the wooded hillsides.

Turkey’s action infuriated Russia, but Moscow’s response has been carefully calibrated. There is little sign it wants a military escalation, or to jeopardise its main objective in the region: to rally international support for its view on how the conflict in Syria should be resolved.

But it clearly wants to punish Turkey economically.

The head of Russia’s tourism agency, Rostourism, said cooperation with Turkey would “obviously” be halted. At least two large Russian tour operators had already said they would stop selling packages to Turkey after Russian officials advised holidaymakers against travelling to its resorts.

Russians are second only to Germans in terms of the numbers visiting Turkey, bringing in an estimated $4 billion a year in tourism revenues, which Turkey needs to help fund its gaping current account deficit.

Medvedev meanwhile said Russia may impose restrictions on food imports within days, having already increased checks of Turkish agriculture products, its first public move to curb trade.

Moscow banned most Western food imports in 2014 when Western countries imposed sanctions on Russia over its role in the Ukraine crisis, leading to supply disruptions as retailers had to find new suppliers and galloping inflation.

The row has also put a brake on new wheat deals between Russia, one of the world’s largest wheat exporters, and Turkey, the largest buyer of Russian wheat.