MOSCOW/UNITED NATIONS - Russian President Vladimir Putin on Wednesday met US Secretary of State Rex Tillerson after complaining of worsening ties with Donald Trump’s administration as the two sides spar over Syria.

Putin received Tillerson at the Kremlin along with Russia’s Foreign Minister Sergei Lavrov after the top diplomats held several hours of talks dominated by the fallout of an alleged chemical attack in Syria.

Despite initial hopes in Moscow of better ties with the US under Trump, the two powers have descended into a furious war of words over the incident and a retaliatory US missile strike against the forces of Moscow’s ally Bashar al-Assad last week.

Russia has slammed Washington’s attack on a Syrian airbase and, as Tillerson met Lavrov, Putin admitted that relations between Washington and Moscow have worsened in the three months that Trump has been in office. “You can say that the level of trust on a working level, especially on the military side, has not improved but most likely worsened,” Putin said in the transcript of an interview with Mir television released by the Kremlin.

“Where is the proof that Syrian troops used chemical weapons? There isn’t any. But there was a violation of international law. That is an obvious fact.”

Meanwhile, US Secretary of State Rex Tillerson battled to overcome differences with his Russian opposite number Wednesday, but they remained split over an alleged chemical attack in Syria after talks in Moscow.  “Despite the quantity of existing problems... there are considerable prospects for joint work,” Russian Foreign Minister Sergei Lavrov told a news conference after talks with Tillerson and Russian President Vladimir Putin.

“Russia is open to this, open to dialogue with the US in all different areas, not only to dialogue but to joint action aimed at results in the areas where this corresponds to the interests of both countries.”

Lavrov said both sides were set on leading an “uncompromising fight against international terrorism”.

He said Moscow was ready to resume a deal with Washington to avoid incidents in Syrian airspace as the two countries lead separate bombing campaigns.

“Today the president confirmed our readiness to return to its implementation on the understanding that the original aims of the air forces of the American coalition are reaffirmed, namely the fight with IS and Al-Nusra,” Lavrov said.

The United States told Russia at the United Nations on Wednesday that is it isolating itself by continuing to support Syrian President Bashar al-Assad, while Britain said its scientists found sarin was used in a deadly toxic gas attack on Syrian civilians last week.

Russia is set to block a push by Western powers at the United Nations later on Wednesday to bolster support for international inquiries into the April 4 toxic gas attack in Syria. It will be Moscow’s eighth veto in support of the Assad government since the Syrian war began six years ago.

“To my colleagues from Russia - you are isolating yourselves from the international community every time one of Assad’s planes drop another barrel bomb on civilians and every time Assad tries to starve another community to death,” US Ambassador to the United Nations Nikki Haley, told the UN Security Council.

During a heated Security Council meeting, Russia’s deputy UN envoy Vladimir Safronkov told the 15-member body that Western countries were wrong to blame Assad for the attack in the town of Khan Sheikhoun.

“I’m amazed that this was the conclusion. No one has yet visited the site of the crime. How do you know that?” he said.

The attack prompted the United States to strike a Syrian air base with cruise missiles and worsened relations between the United States and Russia.

Britain’s UN Ambassador Matthew Rycroft told the Security Council that samples taken from the site of the gas attack, in a rebel-held area of northern Syria, have tested positive for the nerve gas sarin.

He accused Russia of siding with “a murderous, barbaric criminal, rather than with their international peers.”

Safronkov, who demanded Rycroft look at him while he was speaking, responded: “I cannot accept that you insult Russia.”

Haley also accused Iran of being “Assad’s chief accomplice in the regime’s horrific acts,” adding: “Iran is dumping fuel on the flames of this war in Syria so it can expand its own reach.”

Western powers blame the gas attack, which killed scores of civilians - many of them children - on Assad’s forces. Syria’s government has denied responsibility for the attack, which prompted a US strike on a Syrian air base.

Syrian UN Ambassador Bashar Ja’afari said Syria had sent dozens of letters to the Security Council, some detailing “the smuggling of sarin from Libya through Turkey on a civilian air plane by using a Syrian citizen.”

“Two litres of sarin were transported from Libya through Turkey to terrorist groups in Syria,” he said, adding that the government does “not have these weapons.”

UN Syria mediator Staffan de Mistura warned the Security Council on Wednesday that fragile progress in peace talks was now “in grave danger.”