(File image) United Nations High Commissioner for Human Rights Navi Pillay

The UN human rights chief warned on Friday of an all-out war in Syria unless international envoy Kofi Annan's peace plan is implemented and called for an international probe into the killing of more than 100 people in the country.

In an address to the UN Human Rights Council on the massacre in Houla, where UN officials say 49 children were among those killed, Pillay urged the Security Council to refer the massacre to the International Criminal Court in the Hague.

“There is a need for prompt, independent and impartial international investigations into all serious human rights violations in Syria, including those that have occurred in Houla,” Pillay said in remarks delivered by one of her deputies in Geneva.

The council was set to vote later in the day on whether to launch such an investigation.

Syrian envoy Faysal Khabbaz Hamoui blamed the attack on “terrorists,” which is how the Syrian government referred to rebels fighting to overthrow President Bashar al-Assad.

“The situation in Syria might descend into a full-fledged conflict and the future of the country, as well as the region as a whole could be in grave danger,” she added.

Annan was in Lebanon to discuss his peace plan, which calls for a halt to fighting, the withdrawal of heavy weapons from urban areas, a daily humanitarian ceasefire, as well as media access and political dialogue between rebels and government soldiers.

A seven-week ceasefire brokered by Annan appears to be crumbling, raising fears of a civil war.

Russia, Syria's main ally, has come under western criticism for its backing of al-Assad. The Syrian crisis is high on the agenda during talks between President Vladimir Putin with German and French leaders in Berlin and Paris on Friday.

Chancellor Angela Merkel and President Francois Hollande will try to get Putin to drop his opposition to a tough-worded UN Security Council resolution on Syria.

It is going to get harder for Russia to continue its military and diplomatic backing of al-Assad as the violence turns more brutal.

“We consider that recent tragic events constitute an additional reason for the Security Council to refer the situation to the International Criminal Court,” UN rapporteur on arbitrary executions Christof Heyns said.

An EU official on Friday the bloc will push Russia to assume its “responsibilities” in helping to stem the violence in Syria when the two sides meet for a summit in Saint Petersburg at the weekend.

“This is not about pressuring Russia, this is about realising that we have responsibilities to stop the suffering of the Syrian people and find a political way out,” the official said in Brussels, speaking on the condition of anonymity. “We have all seen how much influence and potential leverage Russia has in Syria.”

Rights group Amnesty International urged Putin to halt Russian arms sales to Syria immediately.

British Foreign Secretary William Hague said that “all options” must be considered over the escalating conflict in Syria, though the military option is not on the cards yet.

The Foreign Office in London confirmed that Hague would hold talks with Syrian opposition groups on the sidelines of a peace conference on Somalia in Istanbul.

Meanwhile, at least 15 factory workers were “tortured and then executed” by pro-government forces in the central province of Homs, Syrian activists said.

They added the killings occurred in the village of al-Bueidha where the workers were stopped at an army roadblock and taken to a nearby military post.

Clashes were in the southern province of Daraa, where rebels attacked a military checkpoint, the Britain-based Syrian Observatory for Human Rights reported. – Sapa-dpa