Black smoke billows over the Syrian city of Homs.

Video uploaded to a social media website purportedly shows Homs being bombarded by government forces.

Reuters cannot independently verify the footage.

Authorities are attempting to subdue a year-long uprising.

But residents say the shelling is killing civilians and rebels indiscriminately.

On Monday, Turkey closed its embassy, following the example of many Arab and Western states.

Once a close ally of Syrian leader Bashar al-Assad, it joins a growing number of major powers calling for him to step down.

Jessica Gray, Reuters