A bomb explosion has killed at least 13 people at a crowded market in the Iraqi capital, Baghdad.

About 50 were hurt in the blast, which targeted the popular Ghazil pet market, held every Friday. The bomb was hidden in a box containing birds, police say.

Levels of violence have been falling following the deployment of about 30,000 extra US troops in the Baghdad area since the start of the year.

The US troop surge has targeted Shia militias and al-Qaeda fighters.

The Ghazil market - located in a mixed area - was the target of a bomb attack that killed at least 15 people in January.

The BBC's Jim Muir in Baghdad says bombings used to be a daily occurrence until recently, but are now the exception rather than the rule.

But he adds that attacks are still happening - including a salvo of mortars fired into the heavily guarded Green zone on Thursday.

They are a reminder that unless the security drive is sustained and underpinned by political and economic progress, there could be a return to wholesale violence, BBC correspondent says.

PHOTO CAPTION

An injured man is carried to hospital after the Ghazali pet matket bombing in Baghdad on 23 November 2007