Updated

Authorities found the bodies of six Pakistani men near the Afghan border Sunday who were killed by militants for allegedly spying for the U.S. in an area that has been hit by a wave of drone missile strikes in recent weeks, said intelligence officials and residents.

The bodies of the men — one of whom had been decapitated — were found in North Waziristan, part of Pakistan's lawless tribal region that is dominated by militant groups staging cross-border attacks against U.S. and NATO forces in Afghanistan.

One of those groups, the Haqqani network, an Al Qaeda-linked Afghan Taliban faction, is considered the most dangerous threat to coalition forces and is believed to have helped orchestrate the Dec. 30 homicide bombing at a remote base in Afghanistan that killed seven CIA employees.

In the roughly three weeks following the attack, suspected U.S. drones carried out 12 missile strikes in North Waziristan and neighboring South Waziristan, an unprecedented volley since the CIA-led program began in earnest in Pakistan two years ago. The strikes are part of a broader trend of President Obama's strategy of relying more heavily on the unmanned aircraft to kill militants in Pakistan than his predecessor.

The militants have responded by carrying out a wave of killings targeting people they suspect of helping facilitate the drone strikes. Pakistani intelligence officials have said at least 30 of their operatives were killed in North Waziristan in 2009, many with notes attached to the bodies alleging they were U.S. spies.

The six bodies found Sunday had similar notes, said intelligence officials and residents.

The bodies of five men who had been fatally shot were found on the outskirts of Miran Shah, the main town in North Waziristan, said the officials, speaking on condition of anonymity because they were not authorized to talk to the media.

Authorities found the body of a sixth man who had been beheaded near Mir Ali, another town in the tribal area, said resident Razaullah Wazir.

"This is the fate of American spies," said a note attached to the body written in the local Pashtu language, according to Wazir.

The U.S. does not discuss the drone strikes, but officials have said that they have killed senior Al Qaeda and Taliban leaders in the country.

Pakistani officials protest the strikes as violations of the country's sovereignty, but many analysts believe the U.S. has a secret deal with the government allowing them.

The U.S. has increased its use of the drones in response to Pakistan's reluctance to target militants using its territory to attack coalition troops in Afghanistan. Pakistani officials say they have their hands full battling local Taliban militants waging war against the state.

The Pakistani army launched a major ground offensive in the Pakistani Taliban's stronghold of South Waziristan in mid-October, triggering a wave of retaliatory violence that has killed more than 600 people.

Pakistani police said Sunday that they arrested four people suspected of carrying out a bombing against a Shiite Muslim procession in the southern city of Karachi that killed more than 40 people.

The Pakistani Taliban claimed responsibility for the attack, but Karachi police chief Wasim Ahmed said the four men arrested were members of Jundallah, a militant group based in the city.

Analysts say the Pakistani Taliban has strengthened ties with militant groups throughout Pakistan, a dangerous development.