The Damascus regime has "wilfully" killed thousands of civilians in air strikes that amount to war crimes, Human Rights Watch said on Thursday, as rebels lobbied the major powers for weapons.

G8 foreign ministers gathered for a second day of talks in London after several of them met opposition leaders to discuss their calls for arms to tilt the balance in their fight to oust President Bashar al-Assad.

Overshadowing the talks was a Wednesday statement by the head of the jihadist rebel Al-Nusra Front pledging allegiance to Al-Qaeda chief Ayman al-Zawahiri, which has increased concerns about weapons falling into the wrong hands.

Meanwhile, details emerged of fierce fighting for two towns in southern Syria on Wednesday in which monitors said at least 57 people were killed, among them six children.

Human Rights Watch said bakeries and hospitals were among the civilian targets that been hit by air strikes as the regime resorts to an increasingly indiscriminate use of its air power.

"Individuals who commit serious violations of the laws of war wilfully -- that is intentionally or recklessly -- are responsible for war crimes," the New York-based watchdog said in a report titled "Death from the Skies."

"Syrian government air strikes that have deliberately or indiscriminately killed civilians appear to be part of systematic and widespread attacks against the civilian population that Human Rights Watch previously found amount to crimes against humanity," it added.

Basing its findings on investigations in rebel-held areas of three provinces, the New York-based group documented air strikes on four bakeries and two hospitals, along with other civilian targets.

One hospital in the northern city of Aleppo, the Dar al-Shifa, had suffered at least four attacks alone.

"Air strikes have killed more than 4,300 civilians across Syria since July 2012," HRW said.

Fifty civilians were among 179 people killed in violence on Wednesday, the London-based Syrian Observatory for Human Rights said.

The biggest loss of life was in the southern province of Daraa, where rebels have made gains along the Jordanian border in recent weeks.

"At least six children, seven women, 16 rebel fighters, 16 other unidentified men and 12 army troops were killed on Wednesday, in fighting, shelling and summary executions waged after the army launched an assault on Al-Sanamein and Ghabagheb," the Observatory said.

In London, US Secretary of State John Kerry joined several Western counterparts on Wednesday for talks with opposition leaders about calls for weapons to break the deadlock in an increasingly deadly conflict now in its third year,

A top State Department official said Kerry "didn't promise anything" at the lunch, which was hosted by British Foreign Secretary William Hague.

"We are always considering a variety of options; we are going to continue to aid the opposition, working with them in terms of what they need, in terms of what we're willing to provide," the US official said.

The US and EU are currently providing non-lethal aid, such as communications equipment, and are beginning to distribute food and medical supplies to the Free Syrian Army, but have stopped short of providing weaponry.

Britain has been pushing to amend an EU embargo blocking the supply of arms to the rebels.

Russia is strongly opposed to any attempt to arm the rebels and, as a key ally of Assad, has repeatedly blocked UN Security Council efforts to increase pressure on the Syrian president.

Kerry held talks with Russian Foreign Minister Sergei Lavrov on Wednesday in a bid to find common ground.