FACTBOX: U.S. drone strikes in Pakistan

ISLAMABAD (Reuters) - U.S. drones attacked militants in northwest Pakistan twice on Wednesday, killing more than 30 fighters, security officials said.

The attacks by the pilotless U.S. aircraft were in the South Waziristan region on the Afghan border, a stronghold of Pakistani Taliban leader and al Qaeda ally Baitullah Mehsud.

Here are some facts about the U.S. missile attacks, the controversy they have caused, and a list of some of the more prominent militants killed, according to Pakistani officials.

WHY DOES THE UNITED STATES ATTACK?

Many al Qaeda members and Taliban fled to northwestern Pakistan's ungoverned ethnic Pashtun belt after U.S.-led soldiers ousted Afghanistan's Taliban government in 2001. From their sanctuaries there the militants have orchestrated insurgencies in Afghanistan and Pakistan. The United States and Afghanistan have pressed Pakistan to eliminate the sanctuaries. Apparently frustrated by Pakistan's inability to do so, the United States is hitting the militants itself.

HOW MANY ATTACKS?

The United States has carried out about 47 drone air strikes since the beginning of last year, most since September, killing about 460 people, including many foreign militants, according to a tally of reports from Pakistani intelligence agents, district government officials and residents.

SOME OF THE PEOPLE REPORTED KILLED

January 28, 2008 - A senior al Qaeda member, Abu Laith al-Libi, was killed in a strike in North Waziristan.