WASHINGTON (AFP) - The United States on Tuesday denounced a car bombing in south Beirut which injured at least 53 people, and voiced support for the Lebanese armed forces as they sought to restore calm.

"The US condemns in the strongest possible terms today's bombing in the Bir al-Abed neighbourhood of Beirut which tragically occurred on the eve of the holiday month of Ramadan," State Department spokesman Jen Psaki said.

"We extend our deepest sympathies and concern for civilians wounded in today's attack." The blast, in the stronghold of Lebanon's Shi'ite Hizbollah movement, comes amid spiraling tensions in the country over the war in neighbouring Syria.

Hizbollah forces have joined ranks with Syrian President Bashar al-Assad to fight a mainly Sunni-led opposition movement to oust the Syrian leader, in a conflict which has cost some 100,000 lives.

There are increasing fears that Lebanon - which is hosting hundreds of thousands of Syrian refugees - will be increasingly dragged into the conflict.

"We strongly support the efforts of the Lebanese armed forces and the internal security forces to restore stability and maintain calm in Beirut and throughout Lebanon and we condemn any activity that targets or puts at risk civilians," Ms Psaki added.

She also voiced US concern for the Syrian civilians caught in the crossfire in the rebel-held central Syrian city of Homs, which has been under continuous bombardment by Syrian army forces for the past 11 days, according to a monitoring group.

The "UN estimates that as many as 4,000 civilians remain trapped inside the city without food, water, electricity or medicine," Ms Psaki said.

"We are very concerned as the UN and others are about the inability for humanitarian assistance to get through." Witnesses and activists have said Hizbollah fighters have joined Syrian forces in the assault aimed at driving rebels out of the centre of the city.