(AP) -- U.S. airstrikes against Islamic militants in Iraq have begun -- a day after President Barack Obama authorized those strikes, and humanitarian aid in that country's north.

A Pentagon spokesman says two F/A-18 jets dropped 500-pound bombs on a piece of artillery and the truck towing it. The Pentagon says the militants were using the artillery to shell Kurdish forces defending the city of Irbil (ur-BEEL').

The planes took off from the aircraft carrier USS George HW Bush in the Persian Gulf.

The spokesman says it's not clear how many militants may have been killed in the strike.

In a speech last night in which he threatened to renew U.S. military involvement in Iraq, Obama also announced that U.S. military planes had already dropped food and water to tens of thousands of members of Iraqi religious minorities atop a mountain surrounded by militants.

Defense Secretary Chuck Hagel told reporters today that the U.S. military has enough intelligence resources and assets in place to launch strikes by both manned and unmanned aircraft in the region. >