U.S. and Afghan troops have captured "semi-senior" terrorist leaders along the border with Pakistan, an Afghan government spokesman told The Associated Press on Friday.

The news came as border officials said Pakistan appeared to have blocked roads leading from the showdown in the South Waziristan tribal area, where Pakistani officials said forces had trapped Al Qaeda No. 2 Ayman al-Zawahri (search) in a massive four-day offensive.

The arrests were made somewhere in Afghanistan's lawless border provinces, Jawed Ludin, a spokesman for President Hamid Karzai (search), told the AP. He said it was unclear if those detained had fled the battle in Pakistan and declined to give any details of who might be in custody.

"In recent days, there have been arrests, there have been encounters," Ludin said. "Some of the arrests have included semi-senior leadership within the terrorist elements on the Afghan side, possibly with strong links to Al Qaeda (search)."

The Afghan government considers holdouts from the ousted Taliban regime as terrorists, but Ludin declined to give details or comment on their nationality.

Meanwhile, U.S. forces and their Afghan allies were enforcing "tight security" on the Afghan side of the frontier, senior Defense Ministry official Gen. Atiquallah Ludin said, pledging: "Al Qaeda cannot escape or enter Afghan soil."

Afghan troops on Friday were backing American efforts to hold a cordon along the eastern frontier, strengthened since Thursday by deployments including the dispatch of 250 extra Afghan National Army troops to Khost, north of the standoff, Atiquallah Ludin said.

U.S. troops maintain a base at the Paktika province town of Shkin, just 15 miles from the fighting in Pakistan.

U.S. special operations and CIA forces are believed concentrated in that area as well.

"The latest operations have focused on the border areas," the presidential spokesman said. "Paktika has been a target."

American operatives there are believed to include members of Task Force 121, relocated from Iraq to hunt Al Qaeda after aiding capture of Saddam Hussein (search) in December.

Afghan's border at Paktika province is rugged and lawless — largely out of control of Karzai's U.S.-backed government and rife with Taliban sympathizers.

A sealed border is crucial to the U.S. military's announced hammer-and-anvil strategy for trapping Al Qaeda and Taliban — pinning them against the impermeable barrier of the frontier.

Some travelers from the Afghan side reported military helicopters in the air at the border on Wednesday. By Friday, traffic had dried up on the road to and from Shkin, officials there said.

"The Pakistanis seem to have closed the border," said Mohammed Ghaus, district mayor in the province town of Orgun, site of another U.S. base.

Backed by helicopter gunships and artillery, the Pakistan military operation believed to have cornered al-Zawahri was unfolding in semiautonomous tribal areas traditionally all but off-limits to that country's military.

Afghanistan's government demanded Pakistan do everything in its means to keep Al Qaeda from escaping the Pakistan dragnet — and said Afghanistan would do its part on its side.

Omar Samad, Foreign Ministry spokesman, said the terror suspects should be "either captured or eliminated in some way."

"We hope that they are on the one hand unable to escape, and that all escape is blocked on all sides," he said.

"On our side, we are going to be very attentive that no one attempts to escape the operation in Pakistan and enter Afghanistan, and measures are being taken to prevent that from happening."