Amateur video footage uploaded to a social media website purports to show the Syrian city of Homs under heavy bombardment.

Reuters cannot independently verify the contents of the footage.

Activists say at least 16 people were killed and some 340 wounded on Tuesday during a heavy artillery barrage.

One activist in the Baba Amro district told Reuters that "Several shells are falling each minute", adding that two children were among the victims.

Opposition sources say the offensive has lasted 18 days so far, with government forces backed by tanks and heavy armour advancing on the Sunni Muslim neighbourhood of Baba Amro.

And in the latest sign the 11-month uprising is taking a grip of the Syrian capital as well as the provinces, activists say four people were wounded when security forces opened fire on demonstrators in Damascus overnight.

The International Committee of the Red Cross is in talks with the authorities and opposition fighters for a ceasefire to allow vital aid to reach civilians and for the wounded to be evacuated.

Nick Rowlands, Reuters.