DAMASCUS  - Syrian rebels overran the northern city of Raqa on Monday, scoring their biggest victory since the outbreak of a revolt against President Bashar al-Assad almost two years ago.

In central Syria, insurgents battled a major army offensive to capture rebel-held areas of the city of Homs, a watchdog reported, as the US said it would work to "empower" the opposition.

Reflecting the regional spillover from the conflict, dozens of Syrian soldiers and seven Iraqis were killed in western Iraq as gunmen ambushed a convoy that had crossed the border to escape weekend fighting, a group of Iraqi army officers said.

After days of fierce fighting, the rebels were now in "near-total control" of Raqa, the Britain-based Syrian Observatory for Human Rights said. "This is the first provincial capital in Syria where rebels have made such progress," Observatory director Rami Abdel Rahman told AFP.

"They now have near-total control of Raqa city, except for some regime positions, including the military security and Baath party headquarters."

The Observatory said Al-Nusra Front jihadists fought alongside other rebel groups in the battle for the northern city, which is strategically located on the Euphrates river near the Turkish border. In Raqa, residents destroyed a statue of Assad's father and predecessor Hafez al-Assad, according to amateur video footage distributed by activists.

"Come here Bashar (al-Assad) and see what happened to your father's statue!" cried an unidentified cameraman, as he filmed young residents beating the fallen statue with their shoes, in scenes reminiscent of the 2003 fall of Iraqi dictator Saddam Hussein. The air force then deployed warplanes to bombard an area near the city's presidential palace, according to the Observatory.

The watchdog reported that rebels killed a police chief and captured two senior security officials. "Rebels took the state security chief to Turkey," Abdel Rahman said, noting that the road linking Raqa to Turkey, including the border crossing at Tal al-Abyad, was under rebel control.

Raqa was once home to 240,000 residents, but around 800,000 people forced to flee violence in other parts of Syria have sought shelter there since the start of the conflict, which has claimed more than 70,000 lives, according to the UN.

On Monday alone, at least 105 people were killed across Syria, said the Observatory, adding that 30 of them were civilians.

"The Syrian people lived a historic day today with the liberation of Raqa, the removal of the regime's remnants there," said key opposition group, the Syrian National Council.

The group described the takeover as "a decisive victory in the struggle to overthrow the criminal Assad regime". In the central city of Homs, insurgents fought a fierce army onslaught aimed at crushing rebel enclaves in what activists have dubbed "the capital of the revolution".

The fighting in Homs "is the worst fighting in months and there are dozens of dead and wounded among the assailants," said the Observatory, which relies on a network of medics and activists on the ground for its information. Regular troops backed by pro-regime militiamen attacked the centre of Homs where rebels are holed up, including the Old City and neighbourhoods of Jouret al-Shiah, Khaldiyeh and Qarabees, it said.

And in Iraq, security officers said unidentified armed men ambushed a convoy carrying Syrian soldiers who had entered via the Yaarubiyeh border crossing, the site of weekend fighting, killing 48 Syrians and seven Iraqis.

The soldiers were first transported by the Iraqi authorities to Baghdad from the northern Nineveh province, which borders the crossing, and they were on their way back to be handed over to Syrian authorities on the border with Anbar province in western Iraq when the attack took place, army Lieutenant Colonel Mohammed Khalaf al-Dulaimi told AFP.

Armed men attacked the convoy from two sides with mortar rounds, automatic weapons and mines, killing 42 Syrian soldiers and seven Iraqis. Eight Syrians and four Iraqis were wounded, and three vehicles in the convoy destroyed, he said.

The violence came as US Secretary of State John Kerry said in Riyadh that Washington will work with its "friends to empower the Syrian opposition," though he stressed there was no question of arming the rebels.

Kerry, on his first tour to the region since taking up the post, also met over lunch with Palestinian president Mahmud Abbas, who flew in to Riyadh unannounced late on Sunday.

Kerry 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 the Saudi foreign minister.

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 Gulf Cooperation Council, 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 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. Iran denies these charges.

During his flurry of meetings in Riyadh on Monday, Kerry also held talks with Saudi Crown Prince Salman bin Abdul Aziz, but had no plans for a meeting with King Abdullah, officials said.

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 Ramallah during a visit on March 20-22.

After winding up the Saudi leg of his tour, Kerry headed to Abu Dhabi. He will then head to Qatar.