Islamabad (CNN) -- A convoy of more than 100 vehicles left Islamabad, Pakistan, on Saturday on a march toward South Waziristan to protest U.S. drone attacks.
Pakistani cricketer-turned-politician Imran Khan is leading the march, which includes about 35 members of the U.S. anti-war group Code Pink. The group traveled to Pakistan last week to join the march, group representative Mirza Shahzad Akbar told CNN.
"We are here to say, on behalf of those Americans with a conscience, that we apologize to the people of Pakistan for the killing and suffering," Medea Benjamin, a founder of Code Pink, said at a news conference Thursday.
The U.K. advocacy group Reprieve and former Prime Minister Tony Blair's sister-in-law, Lauren Booth, are also part of the convoy.
But senior government officials warn that the participants will not be allowed into South Waziristan.
"We have already informed the convoy that we will not allow the participants to enter due to security reasons," Shahid Ullah, a senior government official, said.
"The situation in the tribal areas is comparatively good but not suitable for any gathering there," Tashfeen Khan, another government official, said.
"Visiting South Waziristan with foreigners would not be advisable. It can create problems," he said.
While the government has given no assurances for the safety of the march, the Pakistani Taliban has also voiced concern at the convoy entering the volatile region.
"We haven't given any kind of guarantee for the safety of the so-called Peace March planned to visit South Waziristan," said Ihsanullah Ihsan, spokesman for the Pakistani Taliban.
The counterterrorism drone strike program in Pakistan has long been controversial, with conflicting reports on its impact from the U.S. government, Pakistani officials and independent organizations.
American officials insist that the choice and execution of the strikes -- begun under President George W. Bush and ramped up under President Barack Obama -- meet strict standards and that cases of civilian deaths or injuries are extremely rare.
But a study released last month by Stanford Law School and New York University's School of Law said the drone attacks have killed far more people than the United States acknowledges, traumatized innocent residents and been largely ineffective. Civilians account for a significant portion of those killed, the study said.
Meanwhile, fliers were distributed in the city of Tank on Friday from a militant splinter group claiming that Imran Khan was a Jewish American agent and that he would be endangering his own life and those who were traveling with him if he entered the tribal areas.
Convoy participants hope to reach South Waziristan on Sunday and hold a demonstration against drone attacks in northwest Pakistan.
Journalists Amir Iqbal and Nasir Habib contributed to this report