The U.N.-Arab League envoy, Kofi Annan, said he had received a reply from Damascus to peace proposals that he had laid out at the weekend, and wanted further clarifications.
"But given the grave and tragic situation on the ground, everyone must realise that time is of the essence," his spokesman, Ahmad Fawzi, said in a statement from Geneva. Annan will brief the U.N. Security Council on Friday.
With the anniversary of the revolt falling on March 15 and diplomatic efforts to end the violence ineffective thus far, the Syrian army appears to be gaining the upper hand.
It has overrun some opposition strongholds and multiplying attacks others this week, sweeping rebels from the northwestern city of Idlib and sent up to 130 tanks and armoured vehicles to the southern city of Deraa.
"They are hitting the birthplace of our revolution," said a resident of Deraa, who only identified himself as Mohammed for fear of reprisals. The city lies adjacent to Jordan and was the site of some of the first protests against Assad a year ago.
"Houses are being hit with random bombardment from gunfire, RPGs and anti-aircraft missiles," he said of the city, in which 13 civilian and seven rebels were killed on Wednesday, according to the British-based Syrian Observatory for Human Rights.
The Turkish Foreign Ministry said on Wednesday it was working to find out information on the whereabouts of two Turkish journalists who are missing in Syria a week after travelling to Idlib city.
The United Nations estimates some 8,000 people, many of them civilians, have died in the increasingly bloody revolt.
Diplomats have warned that Syria, riven by sectarian divisions, will descend into a Balkans-style civil war unless a political solution can be found. Its economy is already in tatters and its ties with old Arab allies are in shreds.
Official Syrian media accused "armed terrorists" on Wednesday of massacring 15 civilians, including young children, in a pro-government district of the central city of Homs, which has been the focal point of much fighting in recent weeks.
Reports from Syria cannot be independently verified as the authorities deny access to rights groups and journalists.
In the talks at the weekend, Annan presented Assad with a five-point plan to end the fighting. Syria said on Wednesday it had given a "positive" response to the approach.
A Middle Eastern diplomat characterised the reply from Damascus as "not a 'No'". However, adding to the uncertainty, a senior Western diplomat in the region told Reuters that Damascus had rejected Annan's suggestions.
As the army crackdown progresses, a growing number of Syrians are seeking to escape the violence.
The U.N. refugee agency said on Tuesday that some 230,000 Syrians had fled their homes during the past 12 months, of whom around 30,000 have sought safety abroad.
Refugees streamed into Turkey early on Wednesday, saying they had been warned that their villages in the neighbouring province of Idlib would soon be targeted by the army.
"They are bombing Idlib. They are bombing the city. They have tanks and they have rockets," said Abdul Samad, one of the refugees waiting for help at a fog-bound border post.
A Western diplomatic source told Reuters that, while Assad could snuff out pockets of resistance, the rebels were simply pulling back and blending into the background.
"I feel the conflict will last a long time and fear that it will get a lot bloodier than it is now. That's why we have to keep pushing the diplomacy and the sanctions," he said.
The rebels have received little help from the Syrian opposition in exile, the Syrian National Council, who are yet to prove to foreign powers that they can lead and unify Assad's foes inside and outside the country.
On Tuesday, three prominent opposition members resigned from the Syrian National Council, saying they have given up on trying to make the group a more effective player.
Sheikh Nawaf al-Bashir, a well-known SNC member, told reporters on Wednesday that the SNC was "not effective and ... inactive on an international and domestic level," and called for the restructuring of the 270-member group.
Syria comprises a mix of faiths, sects and ethnic groups. Assad's minority Alawite sect dominates the levers of power and holds sway over the Sunni Muslim majority.
Wednesday's reported massacre in Homs was said to have occurred in an Alawite district. On Monday, rebels and government traded blame for the murder of up to 50 people in a mixed area of the city.
The turmoil, coupled with Western sanctions, has cost Damascus billions of dollars in lost revenue from crude oil sales and tourism receipts. The Syrian pound has halved in value, foreign investment has dried up and trade has collapsed.
Yet there is no sign of the Assad family and their allies losing control, or of significant defections from the army.
While Western powers and much of the Arab world have condemned the bloody crackdown, Syria has been able to count on the support of both Russia and China, which have vetoed two U.N. resolutions that were critical of Damascus.
Amnesty International issued a report on Wednesday saying Syria should be referred to the prosecutor of the International Criminal Court. However, only the U.N. Security Council can do that, and it remains divided over what action to take on Syria.
"The testimony presented in this report ... is yet further evidence that torture and other ill-treatment in Syria form part of a widespread and systematic attack against the civilian population," Amnesty said.