A suspected U.S. missile strike killed three people Saturday in a northwest Pakistani tribal region where militants focused on fighting the West in Afghanistan are concentrated, two Pakistani intelligence officials said.

The missile strike was apparently the latest in a lengthy campaign of such attacks by the U.S., which rarely discusses the covert program but has in the past said it has taken out several top al-Qaida operatives. Pakistan publicly opposes the strikes but is believed to secretly aid them.

Saturday's strike occurred in the Babar Raghzai area of North Waziristan and also wounded two people, the officials said. The identities of the dead were not immediately clear. The officials spoke on condition of anonymity because they are not authorized to speak to media on the record.

The area targeted is used by militants from two major factions that are battling U.S. and NATO forces in Afghanistan _ the Haqqani network and the militants of warlord Hafiz Gul Bahadur.

U.S. missile strikes in North Waziristan are sensitive largely because Pakistan has a truce with Bahadur. He agreed to stay on the sidelines as the Pakistani army has waged an offensive in South Waziristan against the Pakistani Taliban, a group that has focused on attacking the Pakistani state.

Missile strikes on his territory could endanger that deal, analysts have said. However, in the past the U.S. has indicated it will not hesitate to launch the drone-fired missiles if it tracks down an important target.

The South Waziristan ground offensive was launched in mid-October, but many leaders of the Pakistani Taliban are believed to have fled to other parts of the lawless tribal belt _ including North Waziristan and the Orakzai tribal regions.

Information from the conflict zones is difficult to independently verify because of restricted access.

The operation in South Waziristan has coincided with a spike in militant attacks in Pakistan, putting the country on edge as more than 500 people have died since October.

An explosion caused by a firecracker late Saturday rattled the southern city of Karachi, wounding 19 people as a group of minority Shiite Muslims staged a procession nearby for the Islamic holy month of Muharram, officials said.

The blast also destroyed a car, but it was unclear if any of the Shiites were hurt. Abdul Rauf, an official with the bomb disposal squad in Karachi, said the firecracker blast left a small crater on the road. Officials were investigating why it was tossed.

Pakistan is mostly Sunni, and extremists from the two sects have often clashed in the country.