Pakistani officials say a suspected U.S. drone strike killed at least three militants in a northwestern tribal region Monday, a day after a similar attack in the same area.

Officials say a missile fired from an unmanned aircraft struck a vehicle traveling through Tapi village in North Waziristan.

On Sunday, officials say seven militants were killed when two missiles struck a militant training compound in the town of Mir Ali.

The semi-autonomous tribal region is used as a sanctuary by the Taliban, al-Qaida, and militants from the Haqqani group blamed for cross-border attacks against international and Afghan troops in Afghanistan.

In other violence Sunday, Pakistani officials said an explosion in southern Sindh province killed three people and wounded at least nine others.

There is no word on what caused the blast in Dadu, but authorities are investigating it as a bombing.  There has been no claim of responsibility.

The U.S. has stepped up drone attacks in northwestern Pakistan since a bomber killed seven CIA employees in neighboring Afghanistan late last year.

U.S. officials do not publicly comment on the drone strikes, which in the past have raised tensions between Pakistan and the United States.  

Pakistan has been wracked by a Taliban-led insurgency that has killed hundreds of people in the past few months.

Some information for this report was provided by AP, AFP and Reuters.