IRAQI soldiers yesterday detained dozens of policemen and closed down a hospital suspected of treating Shiite militiamen in a Baghdad stronghold of cleric Muqtada al-Sadr's Mahdi Army.

Iraqi and US security forces have been battling Mahdi Army fighters in Baghdad since late March. The upsurge in violence has underscored the fragility of Iraq's security at a time when US troops in the capital are reducing their numbers.

The US military announced that the third of five combat brigades sent to Iraq last year to help curb sectarian violence had begun withdrawing. The brigades have been credited with helping to reduce violence.

The military said it expected to complete the withdrawal of about 3,500 troops within the next several weeks under a wider plan to draw down 20,000 troops by July.

The announcement will fuel debate about the ability of Iraq's military to step into the breach. Iraqi soldiers have performed with mixed results in the street battles with Sadr's militia and have relied heavily on US airpower.

Most of the recent fighting has been concentrated in Sadr City, a sprawling slum of about two million people where the anti-US Sadr has a strong following, but Iraqi army units raided Shula district in northern Baghdad on Monday.

The soldiers detained 42 policemen suspected of collaborating with "outlaws" yesterday, an officer of Baghdad's security spokesman Major-General Qassim Moussawi's office said.

Iraq's police are being seen as infiltrated by Shiite militiamen, using the cover of their uniforms to mount attacks.

The soldiers also raided the Mohammed-Bakr Hakim hospital, arresting 35 workers, including orderlies and cleaners, and forced its closure, said hospital chief Dr Yassin al-Rikabi.

Nuri al-Maliki, the Shiite prime minister, launched a crackdown on militias in March.