The Syrian army has moved to retake control of Damascus suburbs from rebel forces, activists say.
They say troops backed by tanks have been shelling rebel-held areas to the east and north of the city.
Dozens of people have reportedly been killed in what activists describe as the fiercest fighting around the capital during the 10 month-uprising.
The major escalation came as the Arab League suspended its month-old monitoring mission in the country.
On Saturday, Secretary General Nabil el-Arabi said: "Given the critical deterioration of the situation in Syria and the continued use of violence... it has been decided to immediately stop the work of the Arab League's mission to Syria."
The league wants the UN Security Council to endorse a peace plan that would see President Bashar al-Assad stepping aside to make way for new elections.
Syria accused it of trying to increase pressure for foreign intervention.
Activists said more than 2,000 troops and 50 tanks had been involved in an attack early on Sunday on eastern suburbs of Damascus, barely 5km (three miles) from the city centre.
There were similar reports from the mountain town of Rankous, about 30km (18 miles) to the north. Anti-government groups said it had become a "disaster zone" with columns of smoke rising from homes hit by shellfire.
The assault reportedly started on Saturday in suburbs where rebels from the Free Syrian Army, which is largely made up of army deserters, have taken position - including Kfar Batna, Saqba, Jisreen, and Arbeen.
Rami Abdul Rahman, head of the UK-based Syrian Observatory for Human Rights, said the fighting was "the most intense near the capital since the uprising began".
"The Syrian regime is trying to finish the uprising militarily now that the case is being taken to the United Nations."
Such reports are difficult to verify because of government restrictions on the media and the suspension of the Arab League monitoring mission.
The diplomatic focus now seems to be switching to the UN Security Council, with speculation that it may vote on a draft resolution on Syria in the coming days.
The council met earlier this week to discuss the document drafted by Arab and European countries.
The draft supports the league's call for President Assad to hand power to a deputy, who would form a national unity government with the opposition within two months.
However Russia, an ally of Mr Assad, has said it will not back the text because it does not rule out foreign intervention.
Mr Arabi is to address the Security Council on Tuesday. He has also been talking directly to Russian officials to try to persuade them to drop their opposition.
In December the UN said more than 5,000 people had been killed since protests against the government of President Bashar al-Assad first erupted last March.
Syria has condemned the league's decision to suspend its mission in the country.
"This will have a negative impact and put pressure on [Security Council] deliberations with the aim of calling for foreign intervention and encouraging armed groups to increase violence," a Syrian official told state TV on Saturday.
The league's mission was established in December to monitor Syria's compliance with a plan to end the bloodshed.
Arab countries voted on Tuesday to extend the mission for another month. Since then conservative estimates say about 200 people have been killed.
As the violence escalated several Arab Gulf countries - including Saudi Arabia - decided to withdraw their observers.
Syrian officials say about 2,000 members of the security forces have been killed in the unrest.