RIYADH - US Secretary of State John Kerry on Thursday briefed wary Gulf ministers 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 talks in Switzerland, Kerry gathered at a Riyadh air base with foreign ministers from the six Gulf Cooperation Council nations.

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

Kerry arrived after talks with Irans Foreign Minister Mohammad Javad Zarif, aiming to seal a nuclear deal with Tehran ahead of a March 31 deadline.

Sunni Gulf nations remain wary about the growing rapprochement between Shiite-dominated Iran and Washington.

But Kerry sought to allay fears, saying Washington remained concerned about Irans 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 told reporters Wednesday before leaving Switzerland.

Iran has provided military assistance to Syria to fight anti-regime forces and to Iraq for the battle against Sunni extremists.

It is also accused of backing Huthi Shiite militiamen who have seized the capital in Saudi Arabias neighbour, Yemen, and paralysed the Western-back 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 thats it," she added.

The so-called P5+1 group of Britain, China, France, Russia, the US and Germany is trying to strike an accord that would prevent Tehran -- Riyadhs 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 Sunni militants from the Islamic State (IS) group, which has captured a swathe of territory in Iraq and Syria.

Riyadh launched air strikes against IS in September but a Western diplomatic source 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 armed opposition from Syria, under a long-planned effort to take on the IS militants.

- Yemens instability -

The jihadists as well as the Western-backed Free Syrian Army are fighting President Bashar al-Assads Syrian regime, which is supported by Tehran-backed Hezbollah troops.

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: "Were 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 wont see peace and security in Syria unless there is a change in the regime in Damascus."

The US militarys top officer, General Martin Dempsey, told American lawmakers on 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.

Yemen, which borders Saudi Arabia, has also been a source of growing regional instability since the Huthi militia seized power in the capital Sanaa in February.

Saudi Arabia and other Gulf states are deeply suspicious of the Huthis, fearing they will take Yemen into Irans orbit.

The US closed its embassy in Sanaa after the Huthi takeover, and is now preparing to base its ambassador out of the US consulate in Jeddah, Saudi Arabia.

Kerry will also meet with new Saudi King Salman, following up on their first talks after the January 23 death of his predecessor Abdullah.

The top US diplomat was part of a heavyweight delegation led by President Barack Obama which held talks in Riyadh five days after Salman acceded to the throne.