RIYADH - US Secretary of State John Kerry briefed wary Gulf ministers Thursday about his latest nuclear talks with Iran, as Washington and its regional allies seek to stabilise a troubled Middle East.

Fresh from three days of negotiations in Switzerland, Kerry gathered at a Riyadh air base with foreign ministers from the six Gulf Cooperation Council nations.

Their agenda was also expected to include ways of reinforcing the battle against militants in Iraq and Syria.

Kerry arrived after talks with Iran's Foreign Minister Mohammad Javad Zarif, aiming to seal a nuclear deal with Tehran ahead of a March 31 deadline. Gulf nations remain wary of a rapprochement between Iran and Washington. But Kerry sought to allay fears, saying Washington remained concerned about Iran's bid to spread its influence in the region.

"For all the objections that any country has to Iranian activities in the region - and believe me, we have objections and others in the world have objections - the first step is to prevent Iran from acquiring a nuclear weapon," he said Wednesday before leaving Switzerland.

Iran has provided military assistance to Syria to fight rebels and to Iraq for its battle against extremists.

It is also been accused of backing Huthi militiamen who seized the capital in Saudi Arabia's neighbour, Yemen, and paralysed the Western-backed government.

"Even as we negotiate, this in no way represents a broader warming of ties, lessening of concerns on our part," insisted deputy State Department spokeswoman Marie Harf. "This is not about a broader rapprochement in any way. This is about the nuclear issue and that's it."

The so-called P5+1 group of Britain, China, France, Russia, the United States and Germany is trying to strike an accord that would prevent Tehran - Riyadh's regional rival - from developing a nuclear bomb.

In return, the West would ease punishing sanctions on Iran, which insists its nuclear programme is purely civilian. Saudi Arabia and other Gulf nations belong to an international coalition brought together by the US to fight the Islamic State group, which has captured swathes of territory in Iraq and Syria.

Riyadh launched air strikes against the extremists in September, but a Western diplomat said the number of Saudi sorties is now "not as many as it has been before."

The kingdom has agreed to launch with the US a facility for training and equipping vetted members of the moderate Syrian opposition under a long-planned effort to take on IS. The militants as well as the Western-backed Free Syrian Army are fighting President Bashar al-Assad's Syrian regime, which is supported by fighters from Tehran-backed Lebanese Hezbollah.

Riyadh, supporting the rebellion, had been angered that the US appeared to sideline moves to reach a political solution under which Assad would give up power. But a senior State Department official insisted: "We're working very closely in Syria with our partners in the Gulf to confront not only ISIL (IS), but to make very clear that we believe that we won't see peace and security in Syria unless there is a change in the regime in Damascus."

The US military's top officer, General Martin Dempsey, told American lawmakers Wednesday it was possible special operations forces could eventually be sent to Syria to back up American-trained rebels.

Washington was quick to stress though that Dempsey was talking about a "hypothetical" situation as moderate opposition forces have not yet been trained.

Moreover, the United States is keeping an eye on Iran’s ‘destabilising’ acts while the two nations try to reach a deal on Tehran’s nuclear programme, Secretary of State John Kerry said Thursday.

‘Even as we engage in these discussions with Iran around this programme, we will not take our eye off Iran’s destabilising actions in places like Syria, Lebanon, Iraq and the Arabian Peninsula, Yemen particularly,’ he told reporters in Riyadh. Kerry, who arrived in Saudi Arabia after three days of talks with Iran’s foreign minister, said he came to update Gulf foreign ministers on the negotiations.

Saudi Arabia and the five other Sunni Gulf nations remain wary about a rapprochement between Shiite-dominated Iran and Washington. But Kerry underscored that ‘we are not seeking a grand bargain’. ‘Nothing will be different the day after this agreement, if we reach one, with respect to any other issues that challenge us in this region, except we will have taken steps to guarantee that Iran will not have a nuclear weapon.’

Daesh is an Arabic acronym for the Islamic State group, which has seized swathes of Syria and Iraq.