Smoke blankets the Syrian border town of Qusair, as heavy fighting rages in the strategic city.

Syrian government forces and Hezbollah fighters were reported to be pressing a sustained assault on the town, while rebels fought off ground and air bombardment.

Video footage of the fighting, said to be shot there on Sunday and Monday, cannot be independently verified by Reuters.

For President Bashar al-Assad, Qusair is a crucial link between Damascus and loyalist strongholds on the Mediterranean coast.

Recapturing the town could also sever connections between rebel-held areas in the north and south of Syria.

Thousands of civilians have been trapped by the violence said United Nations human rights chief Navi Pillay on Monday.

(SOUNDBITE) (English) UNITED NATIONS HIGH COMMISSIONER FOR HUMAN RIGHTS, NAVI PILLAY, SAYING:

"I am extremely concerned at current reports, suggesting that hundreds of civilians have been killed or injured, and thousands may re