Black smoke billows over the Syrian city of Homs.br br Video uploaded to a social media website purportedly shows Homs being bombarded by government forces.br br Reuters cannot independently verify the footage.br br Authorities are attempting to subdue a year-long uprising.br br But residents say the shelling is killing civilians and rebels indiscriminately.br br On Monday, Turkey closed its embassy, following the example of many Arab and Western states.br br Once a close ally of Syrian leader Bashar al-Assad, it joins a growing number of major powers calling for him to step down.