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.

"On Sunday, the highest number of troops and rebels combined were killed since the start of the conflict," said the Observatory's Abdel Rahman.

"We were able to document a death toll of 219 for fighters from both sides for Sunday alone, but we are certain the actual toll is even higher."

In the northern province of Aleppo, a day after rebels captured a police academy in Khan al-Assal, pro-regime daily Al-Watan said insurgents "massacred" 115 policemen in the complex.

"Armed men committed a horrific massacre in the police academy.. leaving 115 dead and another 50 injured," said Al-Watan.

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 42 Syrians and seven Iraqis.

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.