The U.S. military says a bomb attack near a hospital south of Baghdad killed 17 people and wounded 26  Sunday.

The military says the blast is under investigation and was caused by a car bomb.

In Baghdad, the U.S. military says it captured a senior al-Qaida leader linked to deadly vehicle bombings.

Meanwhile, thousands of Iraqis are gathering in Najaf for a protest Monday, on the fourth anniversary of the fall of Baghdad to U.S. forces.  Radical Shi'ite cleric Moqtada al-Sadr urged Iraqis to protest the U.S. presence in Iraq and demand that Iraqi security forces stop cooperating with the U.S. military.

Iraqi military officials say a 24-hour vehicle ban will be enforced in Baghdad for the Monday anniversary to prevent vehicle bomb attacks.

Also Sunday, Iranian officials say they refused to allow a plane carrying Iraqi Prime Minister Nouri al-Maliki to fly through Iranian airspace on a trip to Japan, because permission was not requested in advance.

In another statement Sunday, the U.S. military said a roadside bomb killed four American soldiers Saturday in Diyala province.

And, in Baghdad, military officials said a raid on a Sunni political party safe house netted a cache of weapons and bomb-making materials.  Fourteen suspects were detained.