US Secretary of State John Kerry said during a Gulf tour on Monday that Washington will work to "empower" Syria's opposition, while warning arch-foe Iran that time for talks on Tehran's nuclear ambitions could run out.

Kerry, on his first tour to the region since taking up the post, held talks with Gulf Cooperation Council (GCC) officials and also met in Riyadh over lunch with Palestinian president Mahmud Abbas, who flew in unannounced.

He stressed that there was no question of arming the Syrian opposition, even as his Saudi counterpart Prince Saud al-Faisal insisted on the right of Syrians to self-defence.

The United States will continue to work with its "friends to empower the Syrian opposition," Kerry told reporters during a joint press conference with Prince Saud.

Asked about reports of arms being sent to Syria's rebels from countries such as Qatar and Saudi Arabia, Kerry replied: "The moderate opposition has the ability to make sure that the weapons are getting to them and not to the wrong hands."

However, he added, "there is no guarantee that one weapon or another might not fall in the wrong hands."

The US has so far refused to arm rebels locked in a two-year war against President Bashar al-Assad's loyalists.

Several oil-rich monarchies of the six-member GCC, notably Saudi Arabia and Qatar, have supported the rebellion against Assad, a staunch ally of their regional arch-foe Iran.

The GCC members are dissatisfied at the refusal of President Barack Obama's administration to arm Syrian rebels and its perceived lenient attitude towards Tehran, analysts say.

Kerry said his discussions with Gulf officials had also covered ongoing talks between world powers and Iran on its disputed nuclear programme.

Talks with Iran "will not go on for the sake of talks," he said. "Talks cannot become an instrument for delay that in the end makes the situation more dangerous. So there is a finite amount of time."

"Obama has made it clear that Iran will not get nuclear weapons," said the top US diplomat. "There is a huge danger of proliferation."

World powers negotiating with Iran to rein in its nuclear programme concluded another round of talks in Kazakhstan last week, after putting forward a proposal to ease biting sanctions if Tehran halts the sensitive work of enriching uranium.

"Saudi Arabia supports the efforts to resolve the crisis diplomatically," said the Saudi foreign minister. "We hope that the negotiations will result in putting an end to this problem... the negotiations cannot go on forever."

World powers accuse Tehran of masking a weapons programme under the guise of a civilian atomic drive, charges Iran denies.

Israeli Prime Minister Benjamin Netanyahu warned on Monday that Iran was getting closer to crossing a crucial "red line."

"Iran enriches more and more uranium, it installs faster and faster centrifuges," and it is "running out the clock" on diplomatic efforts to prevent it from obtaining a nuclear capability, Netanyahu said.

During a flurry of meetings in Riyadh on Monday, Kerry also held talks with Saudi Crown Prince Salman bin Abdul Aziz but not King Abdullah.

He also met Abbas in a luncheon meeting which had not been scheduled.

"Well, Mr. President I have been waiting for this meeting and I think you have too," Kerry told Abbas. "That's right, that's right," replied Abbas.

Prior to their meeting, Palestinian envoy in Riyadh, Jamal al-Shawbaki, told the official Voice of Palestine radio that Abbas "will present the Palestinian point of view to the new US administration ahead of Obama's visit".

Obama is due to meet Israeli and Palestinian leaders in Jerusalem and the West Bank city of Ramallah during a visit on March 20-22.

After winding up the Saudi leg of his tour, Kerry headed to Abu Dhabi where he was to meet the Emirati crown prince, Sheikh Mohammed bin Zayed Al-Nahayan, and Foreign Minister Abdullah bin Zayed Al-Nahayan.

On Tuesday, he was to travel on to Qatar.