BAGHDAD, April 24 (Reuters) - A wave of up to 15 explosions resounded in the Iraqi capital Baghdad before dawn on Tuesday, apparently coming from the city's outskirts.

A U.S. military spokesman said he was not aware of any operation taking place at the time.

The blasts sounded like mortar shells or artillery.

While explosions often echo across the city, it is rare for so many to be heard in the early hours of the morning.

U.S. and Iraqi forces are in the third month of a major security crackdown in Baghdad aimed at halting the country's spiral toward all-out sectarian civil war.

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