UN-Arab League peace envoy Kofi Annan has warned that Syria is in danger of slipping into "all-out war".
He said the country's president Bashar al Assad and his regime were key to resolving the 15-month long conflict.
Mr Annan's comments in Doha, Qatar come just days after several alleged massacres in the country.
More than 100 men, women and children were killed in Houla, most of them shot at point-blank range or slashed with knives.
The opposition claimed 13 workers were killed near the western town of Qusair when gunmen loyal to President Assad ordered them off a bus and shot them.
Thirteen other people were shot dead in the eastern town of Assuka.
Speaking at a meeting of the Arab League's ministerial committee on Syria, the envoy said: "The spectre of an all-out war with a worrying sectarian dimension grows by the day.
"The situation is complex and it takes everyone involved in the conflict to act responsibly if the violence is to stop. But the first responsibility lies in the Syrian government and President Assad."
Up to 300 unarmed UN military observers have been deployed in Syria since a ceasefire brokered by Mr Annan came into effect in April as part of a six-point peace plan, which said the army must pull out of towns and cities.
Mr Annan had talks with Mr Assad earlier in the week.
The envoy said: "I told Assad he must act now to implement all points of the plan, and must make bold and visible steps immediately to radically change his military posture and honour commitments to withdraw heavy weapons and cease all violence."
He also said he told Mr Assad to release detainees, open up the country to international humanitarian aid and allow freedom of expression as "this is essential to demonstrate his seriousness to the Syrian people and the international community".
The Arab League's ministerial committee on Syria has called on Mr Annan to set a time frame for his mission.
Qatari prime minister Sheikh Hamad bin Jassim al Thani said it was "unacceptable that massacres and bloodshed continue while the mission is ongoing indefinitely".
Meanwhile, a US government website has published what it said was photographic evidence of mass graves and attacks on civilian areas by Syrian regime forces.
The State Department-operated website published a series of overhead photos showing what it said were mass graves dug following the Houla massacre.
A May 18 photo from Tall Daww, a village near Houla, shows what the government says is a square that appears to be a dirt clearing.
Juxtaposed against this is what American officials said is a May 28 photo of the same square with what appear to be rows of turned up earth, which is labelled as "probable newly-dug graves/trenches".
The satellite photos also showed apparent artillery impact craters near civilian areas of a town called Atarib, and artillery pieces and armoured vehicles near three towns, apparently deployed on Thursday.
The pictures were said to have been taken by commercial satellites - not high resolution military spy spacecraft - and showed what the US said were attack helicopters near the towns of Shayrat and Homs.
UN peacekeeping chief Herve Ladsous said earlier this week that the people who died from artillery and tank fire in Houla were clearly victims of government shelling while the others were most likely killed by "shabbiha" militia loyal to President Assad.
Damascus has blamed the massacre on the opposition, which Mr Assad has tried unsuccessfully for 14 months to crush, killing over 10,000 people in the process, according to the United Nations.
Russia, which has used its veto powers to prevent the UN Security Council from sanctioning Syria, blames Islamist militants for the Houla massacre.
Amid growing fears of a vicious sectarian civil war, British Foreign Secretary William Hague urged opposition groups in Syria to build a united coalition against the regime.
US Secretary of State Hillary Clinton has told Russia that its claimed neutral position in the conflict allows a continuation of violence in the crisis-hit Middle East country.
:: Clashes between pro and anti-Syrian regime gunmen killed at least 10 people and wounded 31 others in the northern Lebanese city of Tripoli, a security official said.
Among the dead were a woman and her son, killed by a rocket in the Bab al Tebbaneh district, a mostly Sunni Muslim community which supports Syria's anti-regime opposition, the official said.
At least five were wounded in Jabal Mohsen, an area mainly populated by pro-Damascus Alawites.