The UN Security Council has unanimously voted to keep its observer mission in Syria for a "final" 30 days.
However, members agreed the mission could be extended further if the use of heavy weapons ends and fighting eases.
The observers - sent to oversee a ceasefire that never took hold - have suspended their work in the face of escalating violence.
With fighting now engulfing areas of Damascus, thousands of Syrians are fleeing into neighbouring countries.
Government forces in the capital are reported to have launched all-out attacks on rebel strongholds.
They have retaken the Midan district and are believed to have launched an assault on the eastern Jubar area.
As President Bashar al-Assad redeploys troops to quell a growing uprising in the capital, rebels have struck at other targets around the country.
On Thursday rebel fighters took control of crossings on Syria's Iraq and Turkey borders.
And on Friday, fierce fighting broke out in Syria's second city of Aleppo, activists said.
The latest violence comes after an attack on Wednesday that killed four senior members of the regime, including national security chief Hisham Ikhtiar, who died from his injuries on Friday.
The UN vote came after hours of intense negotiations among security council members.
Russia had threatened to veto the UK-drafted resolution, but Moscow's ambassador Vitaly Churkin finally backed a revised text.
The resolution will end the observer mission in 30 days. The mandate could then be renewed, but only if Secretary General Ban Ki-moon and the Security Council are able to confirm that both sides are abiding by the terms of the UN-backed ceasefire plan.
Britain's UN ambassador, Mark Lyall Grant, said after the vote: "We have said clearly that it is a final extension unless there is a change in the dynamic on the ground, and in particular that there should be a cessation of use of heavy weapons and that there should be a sufficient reduction in the violence to enable UNSMIS [the observer mission] to carry out its mandate."
US ambassador Susan Rice said it was "unlikely" that the violence in Syria would ease enough to allow a continued UN presence.
She said that Washington's "strong preference" would be for a resolution involving sanctions.
However, Russian President Vladimir Putin warned Western powers on Friday not to take action against Syria outside of the Security Council.
His spokesman Dmitry Peskov was quoted as saying: "In the opinion of the Russian president, any attempts to act outside the UN Security Council will be ineffective and only undermine the authority of this international organisation."
On Thursday, Russia - an ally of Damascus - and China had vetoed a resolution on Syria for the third time in nine months.
Under that Western-backed motion, Syria would have been threatened with non-military sanctions - under Chapter Seven of the UN Charter - if it failed to move troops and heavy weapons from populated areas.
As the situation on the ground in Syria worsened, the UN refugee agency (UNHCR) said that thousands of people were crossing the border into Lebanon.
"The refugee situation has become much more dramatic with the spreading of the violence into Damascus," said UNHCR spokeswoman Melissa Fleming.
"We have figures that there could be anywhere from 9,000 to 30,000 that have fled across the border into Lebanon just in the last 48 hours.
"There may be up to a million Syrians displaced within the country and inside Damascus we're seeing people shifting from neighbourhood to neighbourhood, sleeping in parks, in schools."
The UNHCR says there are already 26,900 registered Syrian refugees in Lebanon, although activists say the real figure is much higher.
A UNHCR spokesman told AFP news agency that about 2,500 people had arrived in Jordan over the past four days, adding to the 35,000 registered Syrian refugees already there.
A spokesman described it as "a steady flow".
About 140,000 Syrians are believed to have fled to Jordan since the uprising against President Assad began in March last year. The Jordanian government is building several refugee camps for them.
Reports also suggested that more than 3,000 Syrians had crossed into Iraq in the past 24 hours.
Some Iraqis, who were returning from Syria, have told the BBC they had been forced to flee their homes near Damascus because of sectarian violence and intimidation. They said they had been targeted by the rebel Free Syrian Army.