Syrian government forces have used helicopter gunships to battle rebels in the capital Damascus, activists said.
Helicopters reportedly fired heavy machine guns during overnight clashes in the neighbourhoods of Qadam and Hajar al-Aswad.
Activists added that Damascus has been relatively quiet since 8am after three days of intense clashes between troops and rebels.
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.
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.
Syria's state-run news agency said troops are still chasing "terrorist elements" who had fled from Nahr Aisha to Midan. 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.
Meanwhile, the United Nations says the number of Syrian refugees who have sought since April has almost tripled to 112,000. The UN refugee agency said women and children make up three-quarters of the Syrians who have been registered or assisted in Iraq, Jordan, Lebanon and Turkey.
UN chief Ban Ki-moon is headed to China as part of a diplomatic push to get Russia and China to back a tougher response to attacks by Assad's regime. Mr Ban's trip comes ahead of a UN Security Council vote this week, with western-backed resolutions calls for sanctions and invoking Chapter 7 of the UN Charter - which can authorise military force.