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BAGHDAD, April 29 (Reuters) - About two dozen loud explosions shook southern Baghdad on Sunday, Reuters reporters heard.

The blasts, several seconds apart, sounded like artillery.

The U.S. military could not immediately confirm the cause of the blasts. U.S. military spokesman Lieutenant-Colonel Christopher Garver said the military sometimes fired artillery in support of operations.

Tens of thousands of U.S. and Iraqi troops have been deployed in Baghdad as part of a 10-week-old security crackdown to combat sectarian militias and insurgents.

While the plan has reduced the number of sectarian killings, car bombs in the capital and across Iraq remain a security concern for military commanders.

Many roadside bombs, the deadliest weapon for U.S. soldiers in Iraq, have exploded recently in southern Baghdad.

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