The United States will not take its eye off Iran's "destabilising" acts in the Middle East, Secretary of State John Kerry said Thursday after talks with Gulf allies wary of an emerging nuclear deal with Tehran.

Kerry, in Saudi Arabia, also said military pressure may be needed to oust Syrian President Bashar al-Assad, who is fighting a civil war against jihadists and Western-backed rebels.

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.

Sunni Gulf states remain wary of a rapprochement between Shiite-dominated Iran and Washington.

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.

"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 said Washington was not seeking a 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."

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

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

The nuclear talks in no way represent "a broader warming of ties, lessening of concerns on our part," deputy State Department spokeswoman Marie Harf insisted earlier.

"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 (IS) jihadist group, which has captured swathes of territory in Iraq and Syria.

Riyadh and its neighbours have carried out air strikes against the Sunni extremists.

Saudi Arabia has also 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.

- Call for ground troops -

Riyadh, supporting the rebellion, was angered that the US appeared to sideline moves to reach a political solution under which Assad would give up power.

Assad has "lost any semblance of legitimacy, but we have no higher priority than disrupting and defeating Daesh and other terror networks," Kerry said, using an Arabic acronym for IS.

"Ultimately a combination of diplomacy and pressure will be needed to bring about a political transition. Military pressure particularly may be necessary given President Assad's reluctance to negotiate seriously."

Saudi Foreign Minister Prince Saud al-Faisal, at the joint press conference with Kerry, called on the US-led coalition to launch a land campaign against the jihadists.

The kingdom "stresses the need to provide the military means needed to face this challenge on the ground," Faisal said.

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.

Yemen has also been a source of growing regional instability since the Huthi militia seized power in Sanaa last month.

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

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

Faisal sat beside Kerry in the meeting with Gulf ministers after returning home from back surgery in the US.

The two men were deep in conversation as they left the air base for historic, mud-walled Diriya on the edge of Riyadh where Kerry met Saudi King Salman at his farm.

Their talks followed an initial meeting after the January 23 death of Salman's predecessor, Abdullah.

Kerry was part of a heavyweight delegation led by President Barack Obama that held talks in Riyadh five days after Salman acceded to the throne.