A suspected U.S. missile struck a car carrying alleged militants in a northwestern Pakistan tribal region Friday, killing three men in the second such attack in less than a day, intelligence officials said.

The strikes are part of the U.S. campaign to rid Pakistan of a creeping militant movement Washington believes is threatening the war effort in neighboring Afghanistan. The rising insecurity inside Pakistan, meanwhile, is prompting the United Nations to relocate about a quarter of its international staff in the country, officials confirmed Thursday.

Both missile strikes occurred in North Waziristan, a lawless tribal region along the Afghan border which is home to several militant groups that tend to focus on attacking U.S. and NATO forces in Afghanistan. The one Friday happened near Mir Ali, a major town in the region, two intelligence officials said on condition of anonymity because they were not authorized to speak on the record.

Shortly after Friday's attack, Taliban arrived at the scene of the attack in the village of Ghundi and moved the bodies to an undisclosed location, the officials said.

The United States has fired scores of missiles from unmanned drones into Pakistan's tribal regions since 2008 in a campaign primarily targeting al-Qaida. U.S. officials rarely discuss the strikes, and Pakistan publicly condemns them, though it is widely believed to aid them secretly.

Pakistan is in the midst of an army offensive against the Taliban in South Waziristan, an operation which has spawned a wave of revenge attacks across the country that has killed more than 500 people since October.

At least 11 U.N. workers have been killed in Pakistan over the past year, and the organization had already reduced its activities in the country's volatile northwest in response to the deteriorating conditions before Thursday's announcement of a partial pullout.

U.N. security managers are seeking a reduction of up to 30 percent in the U.N.'s international staff working inside Pakistan, a U.N. official told The Associated Press on condition of anonymity because security details and negotiations are confidential.

However, the actual number is likely to be lower and will depend on negotiations with the various U.N. agency heads who oversee those workers, the official said. The U.N. employs about 250 international and 2,500 national staff in Pakistan.

The official said an undetermined number of national staff will likely be moved out of Pakistan's North West Frontier Province along the border with Afghanistan, and from the western province of Baluchistan. The U.N. scaled back its operations in Baluchistan in July after a threat by separatists who kidnapped an American aid worker.

In Islamabad, spokeswoman Ishrat Rizvi said around 20 percent of the U.N.'s expatriate workers will either leave Pakistan for six months or be relocated to safer areas within the country. She declined to give specifics on what projects or employees would be affected.

The U.N. began reviewing its operations after an October attack on the World Food Program office in Islamabad killed five people. The goal was to see how it could operate more effectively and safely in Pakistan without disrupting its relief and development aid.

U.N. operations in Pakistan since early 2009 have grown to some $1 billion for the nation's "sustainable development" needs, officials said. Since spring they have also handed out some $475 million in emergency humanitarian aid in northern Pakistan.

Also Friday, Karachi, the country's largest city, came to a virtual standstill after religious and political leaders called for a general strike to protest a bombing that killed 44 people and subsequent riots.

The city's major markets, stores and business centers were closed, along with financial institutions that had already planned to shut because of New Year's Day. Public transportation was halted and gas stations were shut down.

Monday's bombing occurred in the midst of a procession of minority Shiite Muslims during the Islamic holy month of Muharram. Afterward, angry protesters went on a rampage, setting fires to about 2,000 stores that took three days to completely put out.

Interior Minister Rehman Malik, on a visit to Karachi, said investigators were still determining whether the attack was a homicide bombing.

He also questioned the claim of a purported Taliban spokesman, Asmatullah Shaheen, that the militant group was behind the attack. Local news reports on Friday quoted a more prominent Taliban spokesman, Azam Tariq, as denying that the Pakistani Taliban's central leadership had approved the attack, though he did not rule out the possibility that Shaheen's group had carried it out without approval.

In the northwest, a roadside bomb exploded near a car in the Bajur tribal region, killing an anti-Taliban tribal elder and five of his family members, said Nasib Shah, a local government official. Bajur was the focus of a 2008-09 army offensive but still suffers some militant violence. Tribal leaders who support the government against the Taliban are frequent targets of attacks.