Updated

Al Qaeda (search) and Taliban (search) fighters, increasingly pursued by American and Pakistani forces, are on the run or hunkering down rather than mounting a threatened spring offensive of their own, U.S. and Afghan officials say.

More than 50 alleged terrorists have been killed and 163 detained in Pakistan's largest military operation yet against suspected Al Qaeda fighters and local sympathizers in its tribal areas bordering Afghanistan, Pakistani officials said Thursday.

Brig. Mahmood Shah, chief of security in Pakistan's tribal areas, acknowledged that some terrorists might have escaped at the start of the 10-day-old operation.

His comments further fueled speculation that a "high-value" terrorist suspect — said by some officials last week to be Al Qaeda No. 2 leader, Ayman al-Zawahiri (search) — had escaped.

On Thursday, a tape purportedly recorded by al-Zawahiri called Pakistani President Pervez Musharraf (search) a traitor and urged he be overthrown. The audiotape was broadcast by the pan-Arab satellite channel Al-Jazeera.

Afghan government leaders in border provinces told The Associated Press that Taliban and Al Qaeda are fleeing into Afghanistan to escape the crackdown in Pakistan.

But the fighters leaving Pakistan appear to be seeking new hiding places, not launch pads for immediate attacks of their own, the Afghans say, citing cross-border intelligence from fellow Pashtun tribes.

The fugitives "are not in a position to do any terrorist attacks. ... They are just trying to find safe shelter," said Gov. Qulad Khan Mungle of Paktika province, an assessment echoed by other Afghan officials.

Simultaneously, U.S. forces in Afghanistan are two weeks into Operation Mountain Storm — an offensive bent on catching Usama bin Laden and his top Al Qaeda and Taliban allies.

The U.S. operation involved a deployment of hundreds of U.S. troops into one of Taliban leader Mullah Mohammed Omar's home provinces, leaving villagers burying their dead Wednesday from a U.S. ground and air attack.

Most of Operation Mountain Storm has been smaller than past U.S. offensives, with small patrols through border villages, for instance, and hundreds of additional special operations forces reportedly taking a leading, though covert, role in the hunt.

The United States has yet to announce any major successes, but the relative dearth of attacks — against the American military or softer targets — shows the operation is working, a U.S. military spokesman said.

"We're doing a great deal to disrupt operations," the spokesman, Lt. Col. Bryan Hilferty, said in Kabul. "The absence of violence against the Afghan people generally shows how well we're doing."

With spring "we would expect stepped-up activities against the Afghan people and aid agencies — and that's one of the things Mountain Storm is designed to prevent," Hilferty said.

Spokesmen for the Taliban militia, the deposed rulers of Afghanistan, have allegedly promised a spring offensive of their own. Whether the talk was bluster or not, Taliban offensives traditionally come in the spring.

But so far, two weeks into the Pakistan and U.S. operations, retaliatory attacks have been sporadic, and borderline suicidal.

On Wednesday, a lone suspected Taliban opened fire on a U.S. jeep in the southern city of Kandahar, near a U.S. intelligence base at the former home of Mullah Omar. U.S. forces killed him on the spot.

And a man in Uruzgan province, whom Afghan officials described as a Taliban sympathizer, opened fire on a U.S. convoy March 18, killing two U.S. soldiers.

The U.S. military responded by pouring hundreds of troops into the area, a difficult-to-reach central mountain province where Omar spent much of his itinerant childhood and where many believe he has now retreated.

U.S. bombers and A-10 Warthog attack planes leveled the gunman's compound. U.S. forces ended what was a six-day cordon of the area Wednesday, allowing residents to return to bury their nine dead, Afghan officials said, speaking on condition of anonymity. U.S. officials said an Afghan soldier also died in the fighting, resulting in an overall U.S and Afghan death toll of 12.

Reached by satellite phone, a Taliban spokesman claimed Taliban fighters already had launched offensives in Uruzgan province and two others, Kandahar and Zabul.

"We are spreading our operations in all of Afghanistan against U.S. forces," said the spokesman, Mullah Abdul Hakim Latifi.

The Taliban's leaders remain in Afghanistan, including Mullah Omar, or in other areas of Pakistan — not in Waziristan, Latifi claimed. He insisted the Taliban had no contact with Al Qaeda — "wherever they are."

Mungle, the Paktika governor, said tribal informants reported a high but unspecified number of fugitives fleeing from Pakistan into Afghanistan. Other border officials concurred, and said the fleeing fighters now were concerned only with finding hiding places.

Central government officials last week said that authorities arrested "semi-senior" terror suspects in recent days. Afghan Foreign Ministry spokesman Omar Samad said Thursday he knew of no surge in arrests, however.

There are more than 13,000 U.S. troops in Afghanistan. About 2,000 Marines aboard ships in the Persian Gulf plan to join the search for Al Qaeda and Taliban fighters, U.S. defense officials said Thursday. They did not say when the Marines plan to arrive in Afghanistan or how long they would stay.