Pakistani intelligence officials say a U.S. drone strike killed at least four suspected militants in the country's northwest Monday.

The officials say the missiles fired from a pilotless aircraft hit a vehicle near Wana, the main town in the South Waziristan tribal region – a known stronghold of Taliban and al-Qaida-linked militants.

U.S. officials have never publicly acknowledged the use of drone strikes inside Pakistan, but privately they have confirmed their existence to various news outlets. Pakistan has condemned the attacks as a violation of its sovereignty, but the drone strikes are believed to be carried out with the help of Pakistani intelligence.

Elsewhere in Pakistan, gunmen set fire to seven tankers carrying fuel to NATO forces in neighboring Afghanistan. Monday's attack took place in the Khairpur district of Sindh province. At least one person was injured in the attack.

In the capital of the southwestern Baluchistan province, Quetta, two children were killed and three people were wounded when a bomb exploded in a garbage dump.

Also Monday, the Human Rights Commission of Pakistan said the ongoing deadly violence in Sindh's capital, Karachi, is largely a result of dissension between the city's main political groups.

Police say at least 12 people have been killed in various attacks in Karachi since Sunday, with more than 200 people dead from political and ethnic violence in Pakistan's economic hub in July.

Authorities say the killings are part of clashes between the MQM, which largely represents the Urdu-speaking community, and its rival, the Awami National Party , which represents ethnic Pashtuns. Both of those parties and the ruling Pakistan People's Party are believed to have links to armed groups in Karachi.

The commission concluded a three-day fact-finding mission to Karachi on Sunday.