Syrian rebels on Wednesday gave President Bashar al-Assad a 48-hour deadline to comply with an international peace plan otherwise they would renew their battle to overthrow him.

The ultimatum was issued after U.N. observers reported the discovery of 13 bodies bound and shot in eastern Syria, adding to the world outcry over the massacre last week of 108 men, women and children.

The latest developments emphasised how the peace plan drafted by U.N.-Arab League envoy Kofi Annan has failed to stem 14 months of bloodshed or bring the Syrian government and opposition to the negotiating table.

Colonel Qassim Saadeddine of the rebel Free Syrian Army said its leadership had set a deadline of 9 am GMT on Friday for Assad to implement the peace plan, which includes a ceasefire, deployment of observers, and free access for humanitarian aid and journalists.

If it fails to do so ''we are free from any commitment and we will defend and protect the civilians, their villages and their cities," Saadeddine said in a statement posted on social media.

Both sides in the conflict have violated a tenuous ceasefire over the past two months but Assad's forces have been by far the worst offender, according to U.N. monitors.

Outrage at last Friday's massacre in the town of Houla, led a host of Western countries to expel senior Syrian diplomats on Tuesday and to press Russia and China to allow tougher action by the U.N. Security Council.

Major-General Robert Mood, the Norwegian head of the observer mission, said the 13 corpses found on Wednesday in Assukar, about 50 km (30 miles) east of Deir al-Zor, had their hands tied behind their backs. Some had been shot in the head from close range.

Mood called the latest killings an 'appalling and inexcusable act' and appealed to all factions to end the cycle of violence.

He did not apportion any blame but Syrian activists said the victims were army defectors killed by Assad's forces.

Video footage posted by activists showed the bodies face down on the ground, hands tied behind their backs, with dark pools of blood around their heads and torsos.

U.N. peacekeeping chief Herve Ladsous said in New York on Tuesday that the Syrian army and 'shabbiha' militiamen supporting Assad were probably responsible for killing the 108 people in Houla with artillery and tank fire, guns and knives.

The government denied any responsibility and blamed Islamist 'terrorists' - its term for rebel forces.

The uprising began last March with street protests against Assad, who succeeded his late, authoritarian father Hafez al-Assad 11 years ago to perpetuate the family dynasty.

While initially a pro-democracy movement, the struggle has grown into an armed struggle increasingly involving sectarian rivalries pitting the Sunni Muslim majority against the Alawite sect, to which the Assad clan belongs.

Assad's forces have killed 7,500 people since it began, according to a U.N. toll. The government, which says the unrest is the work of foreign-backed terrorists, says more than 2,600 soldiers or security agents have been killed.

PEACE PLAN

Annan, trying to save his peace plan from collapse, told Assad in Damascus on Tuesday that Syria was at a tipping point.

The pro-opposition Syrian Observatory for Human Rights said that more than 100 people were killed the same day. Syria's state news agency said pumping had been halted to an oil pipeline in eastern Syria after a bomb attack on Wednesday.

Diplomats said the U.N. Human Rights Council would meet in Geneva on Friday to consider the Houla massacre, the fourth time Syria has faced such scrutiny since the anti-Assad revolt broke out in March 2011.

Assad has so far proved impervious to international scolding and Western sanctions for his crackdown and has failed to return troops and tanks to barracks, as required by the Annan plan.

However, the U.N. observers sent in to monitor a notional ceasefire were able to verify the horrors in Houla, which produced a wave of world revulsion.

Assad's heavyweight international allies, China and Russia, stuck to their rejection of any intervention or U.N.-backed penalties to force him to change course.

Asked if Western and Arab countries were pressing Moscow to change its position, President Vladimir Putin's spokesman Dmitry Peskov said on Wednesday: "Russia is a country with a consistent foreign policy and any pressure is hardly appropriate."

The West is itself averse to military intervention, although French President Francois Hollande said on Tuesday this could change if the U.N. Security Council backed it. But that is not possible unless veto-wielding members Russia and China allow it.

KICKED OUT

Turkey joined other countries including the United States, Britain, France and Germany in expelling Syrian diplomats in protest at the Houla massacre, saying unspecified international measures would follow if crimes against humanity continued.

Stung by the expulsions, Syria told the Dutch charge d'affaires to leave. She was one of the few senior Western diplomats left in Damascus.

Despite the diplomatic deadlock, Annan, a former U.N. secretary-general and Nobel peace laureate, is pressing on with his mission.

"It is important to find a solution that will lead to a democratic transition in Syria and find a way of ending the killings as soon as possible," he said after talks in Jordan on Wednesday. "With goodwill and hard work, we can succeed."

It is hard to see where a breakthrough might come from.

China reiterated that it opposed military intervention and did not support a forced change of government.

Russia also reasserted its hostility to military action or to any further Security Council measures beyond a non-binding statement condemning the Houla killings.

"We believe consideration in the Security Council of any new measures to influence the situation now would be premature," Deputy Foreign Minister Gennady Gatilov said.

Russia and China have twice vetoed Western-backed Council resolutions condemning the crackdown.

In New York, Annan's deputy Jean-Marie Guehenno told the Security Council that direct engagement between government and opposition was 'impossible at the moment'.

He also expressed serious doubt over the Syrian government's commitment to the Annan plan, a diplomat with knowledge of the closed session said.