DAMASCUS - Clashes broke out between Syrian troops and rebels outside Damascus on Saturday as world powers warned of the country descending into civil war.

And Arab League Secretary General Nabil al-Arabi said he had asked the UN Security Council to take strong action in order to protect civilians in Syria.

"I sent a letter to the UN Security Council asking it to undertake all necessary measures to protect the Syrian people," Arabi said shortly before the opening of a meeting of the ministerial committee on the Syrian crisis.

Asked if he had called for armed action against the regime of Syrian President Bashar al-Assad, Arabi said: "I have not referred to military intervention."

There were no immediate reports of casualties in fighting in the Damascus suburb of Harasta, nor in Douma, where explosions and heavy gunfire amid strikes and protests were reported by the Syrian Observatory for Human Rights.

But one person was killed by gunfire in the Damascus provincial town of Kfar Batna.

And a dissident was killed during raids by regime forces in the central province of Homs, which also took place in the southern province of Daraa, the Britain-based watchdog said.

And tensions spilled into neighbouring Lebanon, as clashes between pro- and anti-Damascus gunmen killed one man and wounded five other people in the northern city of Tripoli.

On Friday, world leaders voiced fears that Syria, wracked by a nearly 15-month uprising against President Bashar al-Assad, stood on the brink of civil war.

At the same time in Geneva, the Human Rights Council ordered an independent probe to hunt those guilty of a massacre last week in Houla that rights chief Navi Pillay said could constitute a "crime against humanity".

Forty-one of the 47-member council backed a call urging an investigation by the Commission of Inquiry on Syria, set up by the council last year to gather evidence on alleged rights abuses.

Russia and China, key allies of Assad, voted against the resolution, which they said was "unbalanced," as it presumed the guilt of the Syrian authorities for the massacre of 108 people, mostly women and children.

Russian President Vladimir Putin, who met separately with German Chancellor Angela Merkel and French President Francois Hollande on Friday, warned that the situation in Syria was "extremely dangerous" and said he saw emerging signs of a civil war.

But he struck a fiery tone in a press conference with Hollande, indicating that Assad's departure would not in itself resolve the crisis.

Hollande kept up the pressure for decisive action, insisting that Assad leaving power was "a prerequisite for a political transition".

After talks with UN chief Ban Ki-moon in Istanbul, British Foreign Secretary William Hague voiced similar concerns.

"Both the secretary general and I -- and also the opposition in Syria -- think that Syria is on the edge of a catastrophic situation... on the edge of an all-out civil war and the collapse of Syria into sectarian strife," Hague said.

Speaking in Lebanon, Annan spoke of his frustration at the slow progress in implementing his six-point peace plan that was supposed to begin with a ceasefire from April 12 but that has been violated daily.

The London-based Syrian Observatory says as many as 2,300 of the more than 13,400 people killed since the uprising against Bashar al-Assad's regime began in March last year have died since the ceasefire began.

The rebel Free Syrian Army said the Annan plan had failed and announced that it would resume "defensive operations" after an ultimatum for the regime to adhere to the plan expired at noon Friday.

The UN ceasefire observer mission in Syria is now at "full strength," with nearly 300 military monitors in the conflict-stricken country, a UN peacekeeping spokesman said Friday.

Because of the worsening violence and Assad's failure to meet commitments under an agreed peace plan, the United States has warned that it may not agree to renew the mission, whose mandate expires on July 20.