Pakistani officials say a U.S. drone has fired several missiles on a vehicle in a northwestern tribal region, killing three suspected militants.

The officials say the drone strike happened Friday in Pakistan's restive South Waziristan region, bordering Afghanistan.

U.S. officials do not publicly acknowledge the use of drone strikes inside Pakistan, but privately have confirmed their existence to various news outlets. Pakistani leaders condemn the strikes as a violation of the country's sovereignty.

Friday's strike comes as tensions rise between Pakistan and the U.S. after claims by top U.S. military officer, Admiral Mike Mullen, that Pakistan's main spy agency backed Haqqani militants who carried out attacks against American targets in Afghanistan.

Top Pakistani officials have denied the U.S. allegations and rejected outside pressure to do more in the war on terrorism.

Pakistani Prime Minister Yousuf Raza Gilani said Thursday the United States should stop blaming his country for regional instability.

On Wednesday, U.S. Secretary of States Hillary Clinton said the U.S. is conducting a final review on whether to designate the Haqqani network a terrorist organization.