Russian troops have deployed along a main road leading into rebel-held areas of the besieged Syrian city of Aleppo ahead of the possible arrival of aid convoys, a local activist said.
Fighting has broken out in the capital, Damascus, in what appeared to be a serious violation of the US-Russia brokered ceasefire.
Insurgents shelled government-held areas in the eastern area of Qaboun, wounding three people, Syrian state media said.
Sana said the shelling violates the ceasefire, which went into effect on Monday.
The Britain-based Syrian Observatory for Human Rights said the fighting is concentrated in the area of Jobar, next to Qaboun.
Mazen al-Shami, an opposition activist near Damascus, said government forces tried to storm Jobar but were repelled by opposition fighters.
Fighters affiliated to al-Qaida and the so-called 'Islamic State' group, who are excluded from the ceasefire, are not present in the area, he said.
The truce has been holding despite some violations in different areas, with the Syrian opposition on Thursday reporting 46 ceasefire violations around the country.
The Observatory reported the first three deaths since the it went into effect.
Outside Aleppo, Syrian forces that had been stationed along Castello road - a main artery into rebel-held areas of the city - were replaced by Russian troops, Rami Abdurrahman of the Observatory said. Aid is expected to enter rebel-held Aleppo later, he said.
Aleppo-based activist Bahaa al-Halaby denied that government troops had withdrawn, adding that humanitarian conditions in the eastern areas of Syria's largest city are deteriorating.
"Humanitarian conditions are very difficult. There are wounded people and others who need food," Mr al-Halaby said. Families in eastern districts receive limited daily rations of bread and there are severe shortages of fruit and vegetables.
Aid deliveries are part of a US-Russia deal that imposed the ceasefire.
Russia's military announced on Thursday evening that Syrian government forces had begun withdrawing from Castello road but did not confirm if Russian troops would be stationed there. The Pentagon said it had no indication of a withdrawal.
Russia's deployment on the road would mark the most overt participation of its ground forces in the Syrian war and underlines the country's role as a major power broker. Russia intervened with its air force on the side of President Bashar Assad's government last year, turning the tide of the war in his favour.
A main opposition group, the Syrian National Coalition, said it rejected "Russian occupation" on the road and UN peacekeepers should run the operation to guarantee the delivery of aid.
Hezbollah's media arm, known as Military Media, said demonstrators from the Shiite villages of Nubul and Zahraa in Aleppo province have started a march toward Castello road to demand that no aid be allowed into eastern Aleppo until aid is sent to two Shiite villages besieged by insurgents in the nearby province of Idlib.
Meanwhile, Russia's deputy foreign minister said the future of Assad is an internal Syrian issue and the US-Russia agreement does not deal with it.
Assad has been accused of war crimes in the civil war and his opponents inside and outside the country have insisted that his departure is a prerequisite for a peace settlement.
Mikhail Bogdanov said in an interview with RIA Novosti that Assad's future is "purely Syrian business".
United Nations officials have said they are awaiting word from Russia and Syrian combatants on both sides that security and monitoring are in place to allow deliveries of humanitarian aid into Aleppo.
OCHA spokesman Jens Laerke said "it is my understanding" that UN officials are waiting for assurances that conditions are safe enough for two convoys of 20 trucks each to proceed from Turkey to eastern Aleppo.
Speaking in Geneva, he said the trucks are in a "special customs zone" on the Turkish border.
He said that the UN does not require authorisation from Syria's government for cross-border aid deliveries.
Jan Egeland, a top UN coordinator of aid for Syria, said it is waiting for assurances on "monitoring arrangements".