US Secretary of State John Kerry has promised 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 Gulf region since taking up the post, held talks with Gulf Cooperation Council (GCC) officials on Monday and also met in Riyadh with Palestinian president Mahmud Abbas, who flew in unannounced.

Kerry stressed 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 (to) the wrong hands."

Kerry will spend day three of his maiden Gulf tour in Qatar on Tuesday, with the Syrian conflict and Iran's disputed nuclear programme still topping the agenda.

The United States has so far refused to arm Syrian 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 US President Barack Obama's administration to arm Syrian rebels and its perceived lenient attitude towards Tehran, analysts say.

The United States praised the holding of elections for a council to run rebel-held areas of Syria's Aleppo province, as well as the new opposition leader's first visit to the country.

Ahmed Moaz al-Khatib's decision to visit the largely rebel-held northern province was a "really courageous choice," State Department deputy acting spokesman Patrick Ventrell said in Washington.

Khatib, head of the Syrian National Coalition, slipped into the country on Sunday as elections were being held for the Aleppo provincial council.

The vote was being held in the Turkish town of Gaziantep for security reasons, but the future body will be based inside Syria.

Syrian rebels overran the northern city of Raqa on Monday, scoring their biggest victory since the rebellion against Assad erupted.

Kerry said his discussions with Gulf officials on Monday 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

The 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 at 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.