International peace envoy Kofi Annan accused Syrian President Bashar al-Assad's forces of atrocities and arbitrary arrests, and said on Saturday he had delivered a blunt message to Assad to act now to implement all points of a peace plan.
Annan, appointed as envoy on Syria by both the United Nations and the Arab League, said the spectre of an all-out civil war was growing daily to the concern of other Middle East countries.
Underlining this fear, nine people were killed and 42 wounded in clashes between Assad supporters and opponents who fired machineguns and rocket-propelled grenades at each other in neighbouring Lebanon's northern port city of Tripoli.
At a meeting with the Arab League, Annan gave a bleak assessment of Syria 15 months on from the start of an uprising against Assad and a week after a massacre of more than 100 people that UN monitors blamed on pro-Assad forces.
"(Assad) must make bold and visible steps immediately to radically change his military posture and honour his commitment to withdraw heavy weapons and cease all violence," said Annan, who met the Syrian leader in Damascus on Tuesday.
"What is important is not the words he uses but the action he takes - now," said the envoy, adding that his message to Assad had been "very direct and frank".
Annan, who has been touring the region, said: "The spectre of all-out civil war, with a worrying sectarian dimension, grows by the day.
"I felt the concerns of Syria's immediate neighbours very acutely in my consultations in recent days."
Lebanese Prime Minister Najib Mikati rushed to Tripoli, about 70km north of Beirut, to try to stop the violence. The army moved into the area with armoured vehicles but did not open fire.
Saturday's death toll was the highest in a single day in Tripoli, raising fears that Syria's unrest could spill over into its smaller neighbour.
In Turkey, nearly 400 more Syrian refugees had crossed the border to escape fighting in Syria's Idlib province, raising the total to more than 24,500, Turkish authorities said.
In Syria, rebels killed six soldiers in the southern province of Deraa and at least eight in clashes on the outskirts of the capital Damascus, the British-based Syrian Observatory for Human Rights said.
The group said two civilians were killed, one during army raids in Damascus and one by gunfire in the city of Homs, the target of a siege by Syrian forces in February and March.
Since the conflict started, Assad's forces have killed 7500 people, according to a UN toll.
The government, which blames the unrest on what it calls foreign-backed terrorists, says more than 2,600 soldiers or security agents have been killed.
Annan said the massacre of men, women and children in the eastern Houla region last week was a terrible crime.
"Worst of all, it is one of many atrocities to have taken place," he said.
"Hundreds of thousands of Syrians are internally displaced. Meanwhile, arbitrary detentions continue, and alongside that, widespread allegations of human rights abuses of all kinds."
Although Annan peace plan looks increasingly forlorn, it appears to be the only option on the table, as foreign governments are reluctant to intervene militarily and Russia is defending Assad on the diplomatic front.
The increasingly sectarian nature of the conflict is fuelling international concern that the conflict could crack open the Middle East's religious and ethnic mosaic if unchecked.
The Assad clan is Alawite, an offshoot of Shi'ite Islam, and he is backed by Iran and many Shi'ites in Lebanon, including Hezbollah. Sunni-ruled Gulf Arab states, led by Saudi Arabia, have spearheaded international opposition to him.
Burhan Ghalioun, of the opposition Syrian National Council, told the Doha meeting Russia must drop its support for Assad.
"With its support of the regime and for Assad remaining, Russia has become part of the problem rather than part of the solution," he said.
Russia has blocked two UN Security Council resolutions condemning Assad and is Syria's main arms supplier.
It backed the government's assertion that the Houla killings were the work of anti-government forces intent on undermining peace efforts.
The United States is urging Russia to distance itself from Assad, a message US Secretary of State Hillary Clinton conveyed to Russian Foreign Minister Sergey Lavrov in a telephone call on Saturday.
"Her message to him was we've got to start working together to help the Syrians with a serious political transition strategy," a senior US State Department official told reporters.
When US officials refer to a "political transition" in Syria, it is code for Assad leaving power.
At the Arab League meeting, Qatar's Prime Minister Sheikh Hamad bin Jassim al-Thani said Annan should set a time limit for his peace mission.
The sheikh, whose country supports arming the mainly Sunni Muslim insurgents fighting to topple Assad, also called on the UN Security Council to put Annan's plan under Chapter 7 of the UN charter, a measure that could authorise the use of force.
Arab League foreign ministers called for an increase in pressure on Assad but stopped short of backing military intervention.
"The decision that was taken... has no military elements at all, we have to be clear about that," Arab League chief Nabil el-Araby said.
The new anti-Assad measures included calling on Arab satellite companies to stop airing Syrian television channels, Kuwait's Foreign Minister Sheikh Sabah Khaled al-Sabah said.
The Arab League also urged the fragmented Syrian opposition to meet in Cairo in an attempt to form a united front.
Annan's plan involves withdrawing heavy weapons and ceasing violence, releasing detainees, letting in foreign humanitarian aid, and allowing people to protest peacefully.
Nearly 300 military observers are deployed in Syria to monitor the now-collapsed ceasefire.
"But we do not have what this was set up to achieve - an end to the appalling violence and abuses, and the launch of a political process for a transition that meets the legitimate aspirations of the Syrian people," said Annan.