Nearly a week of political and ethnic violence has killed at least 91 people in Pakistan's largest city.

Police say the bodies of at least 11 people who were shot or tortured to death have been brought to government hospitals in Karachi since Sunday.

Pakistani Prime Minister Yousuf Raza Gilani traveled to the southern port city and held talks with government officials in Sindh province Monday. The prime minister said strict action should be taken against those responsible for the violence.

Hundreds of people have been killed in Karachi in recent months, in one of the worst waves of unrest there in years. Many of the killings are blamed on ethnic gangs linked to rival political factions such as the majority Urdu-speaking Muttahida Qaumi Movement and the Awami National Party of Pashtun migrants.

In Pakistan's northwest, local security officials say a U.S. drone strike has killed four suspected militants in the North Waziristan tribal region.

Pakistani officials say two missiles hit a vehicle Monday near the town of Mir Ali.

North Waziristan is a known refuge for Taliban and al-Qaida-linked militants and the United States has pressed Pakistan to launch an offensive in the region.

U.S. officials do not publicly acknowledge the use of drone strikes inside Pakistan, but privately have confirmed their existence to various news outlets. Pakistan has condemned the attacks as a violation of its sovereignty, but the strikes are believed to be carried out with the help of Pakistani intelligence.

Also Monday, authorities say gunmen set fire to at least 18 trucks carrying fuel for NATO forces in neighboring Afghanistan. There were no reported casualties in Monday's attack in the southwestern province of Baluchistan.