The U.S. military says an explosion has rocked an American military base in the northern Iraqi city of Mosul, causing many casualties.
Military officials say they are investigating the cause of the blast, which shook the base at midday Tuesday local time. Officials have not said which base was hit, but the U.S. Army's Task Force Olympia is based in Mosul.
Insurgents have stepped up their attacks in Mosul recently, after many of them fled their former stronghold in Fallujah.
Meanwhile, U.S. warplanes bombed suspected insurgent targets in the town of Hit, northwest of Baghdad Tuesday. Hospital officials say four people were killed.
At a news conference in Washington Monday, President Bush acknowledged that insurgent attacks are having an effect in Iraq, and that U.S. efforts to train Iraqi forces have produced only mixed results.
Information for this report is provided by AP and Reuters.