Pakistani security officials say a U.S. drone strike has killed at least five Islamist militants in a restive tribal region near the Afghan border.

The two missiles fired late Monday from the unmanned American spy aircraft slammed into a compound in the Mir Ali area of North Waziristan. Some of the dead were believed to be foreign fighters, but it is unclear what nationality.

There has been a dramatic increase in U.S. drone strikes in Pakistan since May.

The drone program is extremely controversial in Pakistan, where residents view it as a violation of their sovereignty and say it often results in civilian deaths. But U.S. officials believe it is vital to combating militants that threaten the United States.

Pakistan's semi-autonomous northwestern tribal belt is considered the main hub of Taliban and al-Qaida-linked militants plotting attacks on the West and in Afghanistan.