Syrian government forces have used helicopter gunships to battle rebels in the capital Damascus, activists said.
Helicopters fired heavy machine guns during overnight clashes in the neighbourhoods of Qadam and Hajar al-Aswad, the activists said.
They said Damascus has been relatively quiet since 8am after three days of intense clashes between troops and rebels.
The fighting has been the most sustained and widespread in the capital since the start of the uprising against president Bashar Assad 16 months ago.
The Britain-based Syrian Observatory for Human Rights and Damascus-based activist Maath al-Shami said the fighting concentrated in Kfar Souseh, Nahr Aisha, Midan and Qadam.
Syria's state-run news agency said today that troops are still chasing "terrorist elements" who fled from Nahr Aisha to Midan.
The fierce clashes are the latest sign that Syria's conflict is fast descending into a civil war that is moving ever closer to the heart of president Bashar Assad's regime.
Government forces have already thrown tanks and armoured personnel carriers into the fight in the capital, but the use of airpower reflected the intensity and seriousness of the clashes.
The UK-based Syrian Observatory for Human Rights and Damascus-based activist Maath al-Shami said the fighting was concentrated in Kfar Souseh, Nahr Aisha, Midan and Qadam.
Syria's state-run news agency said troops are still chasing "terrorist elements" who had fled from Nahr Aisha to Midan.
An amateur video showed two armoured personnel carriers with heavy machine guns on top along with troops who were said to be advancing in an empty road toward Midan on Tuesday.
Activists have dubbed the fighting in the capital the "Damascus Volcano" in what appears to be an attempt to bring the fighting into Syria's seat of power.
The clashes are the most sustained and widespread in the capital since the start of the uprising against the Assad regime in March last year and a crackdown that activists say has claimed the lives of more than 17,000 people.
In the past, clashes happened at night in the capital. Now, the fighting rages during the day.
Damascus - and Syria's largest city, Aleppo - are both home to elites who have benefited from close ties to Assad's regime, as well as merchant classes and minority groups who worry their status will suffer if Assad falls.
As the violence has spiralled out of control, diplomatic efforts to halt the bloodshed have faltered, with world powers still deeply divided over who is responsible and how to stop the fighting. The US and many Western nations have called on Assad to leave power, while Russia, China and Iran have stood by the regime.
UN chief Ban Ki-moon was headed to China on Tuesday as part of a diplomatic push to get Russia and China to back a tougher response to attacks by Assad's regime. Ban's trip comes ahead of a UN Security Council vote this week. A Western-backed resolution calls for sanctions and invokes Chapter 7 of the UN Charter.
A Chapter 7 resolution authorises actions that can ultimately include the use of military force, which US administration and European officials - for now - are playing down as a possibility.
Mr Ban is due to hold talks with Chinese president Hu Jintao on Wednesday, with Syria expected to top the agenda.
UN special envoy Kofi Annan, meanwhile, is set to meet Russian president Vladimir Putin in Moscow to discuss the conflict.
In Tehran, Iranian foreign ministry spokesman Ramin Mehmanparast warned against foreign intervention in Syria. Iran, one of Syria's strongest allies, offered to mediate and host a meeting between the Syrian government and rebels to help resolve the conflict.Reuse content