US senator John McCain, one of the strongest advocates of US military aid for the Syrian opposition, has crossed into Syria to meet with rebel leaders.

Mr McCain's visit came as British foreign secretary William Hague announced that the European Union (EU) has agreed to lift an arms embargo in order to be able to supply weapons to the rebels.

Senator McCain's spokesman Brian Rogers confirmed the senator slipped across the border in an unannounced visit lasting several hours, before returning to Turkey.

Key points John McCain crosses from Turkey into Syria to meet with rebels.

John McCain crosses from Turkey into Syria to meet with rebels. Rebels reportedly asked for heavy weapons, no-fly zone and air strikes on Syrian forces.

Rebels reportedly asked for heavy weapons, no-fly zone and air strikes on Syrian forces. McCain spokesman says unannounced visit lasted several hours.

McCain spokesman says unannounced visit lasted several hours. US secretary of state John Kerry meets with Russian counterpart in Paris.

The visit by the 2008 Republican presidential nominee followed an overwhelming US senate panel vote last week to send weapons to forces fighting against the Syrian government.

"I can confirm the trip. It was today," Mr Rogers said, adding that no other lawmakers travelled with Senator McCain into Syria.

Photos posted on Instagram by the Syrian Emergency Task Force showed Senator McCain with men who appeared to be young Syrian rebels.

US news website The Daily Beast reported that Senator McCain entered the country with General Salem Idris, leader of the Supreme Military Council of the Free Syrian Army.

During the talks, rebel leaders asked for increased US support to the Syrian opposition by providing heavy weapons, establishing a no-fly zone and carrying out air strikes on Syrian regime forces, The Daily Beast reported.

"The visit of Senator McCain to Syria is very important and very useful especially at this time," the report quoted General Idris as saying.

"We need American help to have change on the ground; we are now in a very critical situation."

EU lifts embargo to arm Syria rebels

Senator McCain is among the sharpest critics of the Syria policy of US president Barack Obama, who has signed off on Washington providing non-lethal and humanitarian aid to the rebels but has so far refused to provide arms.

EU governments have reached a political deal to extend all financial and economic sanctions to Syria for one year, while agreeing to end the arms embargo to help the opposition.

"This is the outcome that the United Kingdom wanted," Mr Hague said, adding that the decision had been "difficult" for those EU nations staunchly opposed to delivering arms they believed would serve only to fuel the conflict.

"I think it is the right decision.

"It will support political progress on Syria and our attempts to bring together a Geneva conference."

Mr Hague stressed that Britain had "no immediate" plans to supply weapons to rebels fighting President Bashar al-Assad's regime.

Mr Hague said Britain saw only a political solution and a diplomatically-supported solution for Syria, but that the ground-breaking decision "sends a very strong message from Europe to the Assad regime of what we think of the continued brutality and murder and criminality of this regime."

"While we have no immediate plans to send arms to Syria, it gives us the flexibility to respond in the future if the situation continues to deteriorate and worsen," he said.

Mr Assad's forces have mounted intense assaults on the rebels over the past week, particularly in the insurgent bastion of al-Qusair, in central Syria.

Earlier this month, US ambassador to Syria Robert Ford - who was recalled from Damascus in October 2011 over concerns for his safety - met with a Syrian rebel chief at a border crossing with Turkey.

Australia's Foreign Minister Bob Carr also visited the region last week, meeting with refugees at their camp in Lebanon's Bekaa Valley, on the Syrian border.

ABC/AFP