Kofi Annan, the international envoy, has said his six-point peace plan for Syria is a "possible last chance to avoid civil war".
Annan, the UN-Arab League envoy tasked with bringing an end to the violence, said on Tuesday that world powers shared a "profound concern" that Syria's violence was escalating into civil war.
He said they had pledged to deploy 300 truce monitors to Syria by the end of the month.
Addressing the UN Security Council from the Swiss city of Geneva via video-link, Annan said there had been "a spate of bombings that are really worrying'' and that the UN mission, mandated with monitoring a April 12 ceasefire in the country, was "the only remaining chance to stabilise the country".
The peace bid was not an "open-ended" opportunity for Syrian President Bashar al-Assad, Annan said.
"There is a profound concern that the country could otherwise descend into full civil war, and the implications of that are frightening," he said.
Annan said failure to prevent a civil war "will not only affect Syria, it will have an impact on the whole region".
He also said that there had been "some decrease in military activities, but there are still serious violations in the cessation of violence that was agreed".
Annan told the UN Security Council he planned to carry out a second trip to Syria since he was named special envoy.
There was no indication yet of the timing of the trip, his spokesman, Ahmad Fawzi, told AFP news agency, but it would "certainly not" be in coming days.
"I would say weeks but all depends on what we hear from Damascus," he said in a written statement.
There are only around 60 UN observers in Syria, although the team is expected to grow to 300 in the coming weeks.
Annan said: "Government troops and armour are still present though in smaller formations. There have been worrying episodes of violence by the government, but we have also seen attacks against government forces, troops and installations."
Responding to Annan's remarks, Bashar al-Jafaari, Syria's ambassador to the UN, said "there is a positive trend on the ground" mainly due to the government's commitment to fulfil Annan's plan.
"However, we are still facing some Arab, regional and international powers who are deploying huge efforts in order to topple and undermine the mission of the observers," he said.
"So the big problem we are facing right now is the smuggling of weapons, sponsored by some Arab, regional and
international powers into Syria."
Jaafari accused Turkey of allowing armed groups to infiltrate into Syria and create violence.
"Turkey is harbouring some dissidents from the military as well as armed groups on its own territory, and these armed groups are infiltrating the Syrian-Turkish border every day, committing murders, crimes against the police and the security forces as well as against the army ... and then they withdraw and they go back and retreat into Turkish territory," he said.
Al Jazeera's Kristen Saloomey, reporting from the UN headquarters in New York, said: "Syria's ambassador was very clear to point the finger to foreign fighters in the country. He blamed them for breaking the ceasefire and blamed countries like Turkey, Saudi Arabia and Qatar for encouraging them to do so."
'Areas of civil war'
The latest diplomatic developments came as the International Committee of the Red Cross (ICRC) said the situation in some parts of Syria already resembled a civil war.
Jakob Kellenberger, head of the ICRC, told Reuters news agency on Tuesday that the conflict in Homs and in the province of Idlib this year met the agency's three criteria of a non-international armed conflict: intensity, duration and the level of organisation of rebels fighting government forces.
In-depth coverage of escalating violence across Syria
“It can be a situation of internal armed conflict in certain areas: an example was the fighting in [the neighbourhood of] Bab Amr in Homs in February," Kellenberger said, making it clear that the criteria were not met in the entire country.
Earlier, Recep Tayyip Erdogan, the Turkish prime minister, said he was losing hope for a solution and urged the UN to bolster its observer mission to up to 3,000 rather than the 300 authorised under the April 12 Security Council resolution.
"The UN should bolster its mission to Syria with up to 3,000 observers to give a full picture of the situation in the country," he said on Monday while on a visit to Rome, Italy.
"We need 1,000, 2,000, maybe 3,000 observers, a major mission so they can visit the whole country and see what is happening.
"We support the Annan plan but if someone were to ask me what my hopes are, I would say I have lost hope."
Activists say that more than 10,000 people have been killed across Syria since the uprising against Assad broke out in March 2011.
The Local Co-ordination Committees, an opposition activist network, said 20 people had been killed across the country on Tuesday, including eight people in Idlib.
The unrest has persisted despite the parliamentary elections held on Monday as part of the government’s pledge to implement reforms.
The opposition boycotted the vote, denouncing it as a sham while the US said it was "bordering on ludicrous".
The Syrian government said turnout was high in the election that marked the first time the country held a vote since the adoption in February of a new constitution allowing for multi-party polls.
"Millions of Syrians defied terrorism and chose their representatives in parliament," the pro-government daily al-Watan newspaper said as it put the voter turnout at 60 per cent.