A homicide bomber attacked the main border crossing for convoys ferrying supplies to U.S. and NATO troops in Afghanistan on Thursday, killing at least 19 security officers, officials said.

The strike will raise fears the Pakistani Taliban is regrouping and making good on its word to carry out revenge attacks following the slaying of its leader, Baitullah Mehsud, in a CIA missile strike earlier this month.

Also in the border region, two U.S. missiles hit a suspected militant compound, killing six people, the latest in a string of such attacks, intelligence officials said.

Pakistan's lawless border with Afghanistan is a main front in the battle against Al Qaeda and the Taliban, who are destabilizing both countries. Under heavy U.S. pressure, the Pakistani military has launched ground offensives and air attacks on the insurgents in recent months, but much of the region remains under militant control.

The homicide attacker walked up to a group of border guards outside their barracks at the Torkham checkpoint in the Khyber region and detonated his explosives, local police officer Sadiq Khan said. The victims were breaking their daylong fast during the Islamic holy month of Ramadan. The border had closed for the day a few hours earlier.

Ali Raza, an official in the administration office, said he heard a huge explosion in the building next door.

"We rushed out and saw destruction all around," Raza said.

At least 19 people were killed and 20 wounded, according to Fazal Akbar, the head doctor at Landi Kota hospital, where all the victims were taken.

The Torkham checkpoint marks the main border crossing from Pakistan's Khyber Pass into Afghanistan.

U.S. and NATO troops in landlocked Afghanistan rely on the supply line for up to 75 percent of their fuel, food and other logistical goods. Thousands of civilian vehicles also use the route.

Militants have targeted NATO conveys and bombed bridges along the route in the past — cutting off supplies briefly last year — but Pakistani military analyst Hasan-Askari Rizvi said he doubted that was the motive in Thursday's attack.

"My own feeling is that this was aimed at those government forces rather than stopping supplies," he said.

Pakistan's government dispatched paramilitary forces to escort supply convoys through the Khyber Pass after several attacks last year, and there has not been a major assault on a convoy for nearly six weeks.

No one claimed immediate responsibility for the bombing, but the Pakistani Taliban will be a likely suspect.

Rizvi said the militants could be trying to prove they are still unified and effective after Mehsud was killed on Aug. 5. The Pakistani Taliban only acknowledged he was dead on Tuesday and named 28-year-old Hakimullah Mehsud as his successor after reports of a power struggle.

Another U.S. missile struck Thursday in Mehsud's stronghold of South Waziristan, said two Pakistani intelligence officials on condition of anonymity because they were not authorized to speak to the media.

There were conflicting claims as to the identities of the dead.

One of the intelligence officials said they were believed to be militants from Uzbekistan. Pakistani Taliban spokesman Azam Tariq called the Associated Press soon after the attack and said that women and children were the only victims.

Neither claim could be independently verified. The attack was in a remote area of the tribal region, which is off-limits to journalists and largely under Taliban control. In the past, both the government and Taliban have passed on information that was not true.

The United States has launched more than 40 missile strikes from unmanned planes on Al Qaeda and Taliban targets close to the Afghan border since last year, reportedly killing several top commanders, but also civilians. It does not comment on the attacks.

The missiles are fired from CIA-operated drones believed to be launched from Afghanistan or from secret bases inside Pakistan. They are reported to be piloted by operatives inside the United States.

The Pakistani government publicly protests the attacks, though is assumed to be cooperating with the strikes and providing intelligence for them. It has called on Washington to give the technology for such attacks to Islamabad because its military is capable of using the drones.