Syrian forces have fired heavy machine guns and launched at least one airstrike on a neighbourhood in the central city of Homs that rebels claimed to have seized.
Monday’s fresh fighting raged in and around the Baba Amr district, sending civilians in nearby neighborhoods fleeing the area.
Baba Amr, a poor neighbourhood on the southwest side of Syria's third largest city, was a centre of opposition to President Bashar al-Assad early in the uprising that began two years ago.
Last year, government forces besieged Baba Amr for a month before rebel forces withdrew and the government seized control on March 1. Hundreds of people were killed in the violence, including two foreign journalists.
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It was unclear how much of the neighbourhood rebels had seized on Sunday or continued to hold on Monday.
An activist in Homs reached via Skype on Monday said rebel fighters made a surprise entry into Baba Amr the previous morning and quickly took over seven government checkpoints in and near the neighbourhood, seizing a good part of the area.
The government struck back on Monday, carrying out an airstrike, shelling the area and sending reinforcements, he said.
'Symbol of the revolution'
Another activist from Baba Amr, who is now in Lebanon, said rebels had taken the whole neighbourhood and were focused on holding it despite the government's harsh counter attack.
He said it was very important to the regime militarily and as a symbol, so rebels are excited that this symbol of the revolution has been retaken. Both activists spoke on condition of anonymity out of concerns for their safety.
An activist posted video online that appeared to show dozens of civilians, most of them women and children, fleeing the neighbourhood.
The Syrian state news agency said the army had responded to "terrorists who sneaked into Baba Amr in Homs and committed crimes against people," killing a number of them. It said the army continued to pursue those remaining in the area.
The Syrian government blames the civil war on a foreign-backed conspiracy, calling the rebels "terrorists" bent on destroying the country.
In Brussels, European Union foreign ministers met to discuss ways to end the violence in Syria. They were expected to discuss ending the EU's blanket arms embargo against the country to make it easier for sympathetic nations to send arms to the rebels.
Many EU diplomats, however, say more weapons will only exacerbate the situation.
Syrian General Salim Idris, a defector who now heads the rebels' High Military Council, travelled to Brussels last week to plead for arms from the international community.