Russian President Dmitry Medvedev has warned that Kofi Annan represented the last chance for avoiding a civil war in Syria and offered the UN-Arab League envoy Moscow's full support.
"This may be the last chance for Syria to avoid a protracted and bloody civil war," Russian agencies quoted Medvedev as telling Annan at a meeting on Sunday, adding that Russia would provide "full support at any level" for his mission.
At the meeting, Foreign Minister Sergei Lavrov "underscored the need to end violence from all sides and establish a broad Syrian political dialogue. ... He called on the special envoy to work actively toward that aim with both the authorities and the opposition," a statement said.
Lavrov also said that to support Anna's mission, nations must refrain from interfering in Syria affairs or taking sides in the confrontation between the government and opponents.
The Russian warning came as US President Barack Obama pledged to send "non-lethal" aid to the Syrian armed opposition in the most overt show of US support for the fighters to date.
The decision seems certain to irritate Russia following its fierce condemnation of the West's calls on President Bashar al-Assad to step down.
Obama said at talks with Turkish Prime Minister Recep Tayyip Erdogan ahead of a nuclear security summit in Seoul that they agreed "there should be a process" of transition to a "legitimate government" in Syria.
A top US national security official said the delivery of medical aid and other urgent supplies would top the agenda of a "Friends of Syria" meeting scheduled for April 1 in Istanbul.
Both Russia and China have used their veto rights as permanent members of the UN Security Council to block efforts to condemn Assad.
Annan, the former UN chief, will be carrying with him Assad's answer to a peace plan under which Syria could begin a "political transition" to a representative government, with no specifically defined role for Assad.
The initiative calls for Assad to withdraw troops and heavy weapons from protest hubs, a daily two-hour humanitarian ceasefire, access to all areas affected by the fighting and a UN-supervised halt to all clashes.
Blasts meanwhile once again rocked Syria's flashpoint city of Homs as government forces pressed on with their assault on protest hubs while the armed opposition countered by attacking a military base in Damascus province.
Opposition activists said at least 27 people were killed in several districts in Syria on Sunday.
Activists in Homs reported heavy shelling of al-Khaldiyeh, al-Hamidiyeh and other old neighbourhoods in the city in an attempt by the government to regain control of pockets of armed resistance.
A large number of al-Qusair residents in Homs reportedly fled their homes into neighbouring villages after their town was heavily bombarded by artillery fire.
In the southern town of Nawa, the Local Co-ordination Committees (LCC) activist network said "tanks have entered the main streets, and heavy gunfire by regime forces is reported".
Nawa is in Deraa province, where the popular uprising against President Bashar al-Assad's rule erupted in March 2011.
Elsewhere in the country on Sunday, near the northern border with Turkey, rockets were fired into the town of Aazaz as helicopters flew overhead, according to activists.
LCC said opposition fighters of the Free Syrian Army blocked a highway used by the military for reinforcements and supplies to Aazaz, which has been the scene of fierce clashes for the past few weeks.
The state news agency SANA said an "armed terrorist group" attacked a gas pipeline in the eastern province of Deir ez-Zor. Several such attacks have caused explosions in Deir ez-Zor and Homs provinces.
At least 60 people were reportedly killed across Syria on Sunday, activist said.
Human Rights Watch (HRW) said government forces in Idlib had resorted to using civilians as human shields to protect themselves from attacks by opposition fighters.
Citing witnesses and YouTube videos, the US-based rights group accused the army and pro-government militia known as shabiha of forcing people to march in front of them as they advanced on opposition-controlled towns in the province.
"By using civilians as human shields, the Syrian army is showing blatant disregard for their safety," Ole Solvang, HRW emergencies researcher, said in a statement.
"The Syrian army should immediately stop this abhorrent practice."
The tactic had reportedly been used in the towns of Al-Janudyah, Kafr Nabl, Kafr Ruma and Ayn Laruz.