The top U.S. commander in Afghanistan questioned Pakistan's commitment to fighting Taliban and Al Qaeda (search) militants along the border, saying Monday that appeasing extremists will only put off an inevitable battle.

Lt. Gen. David Barno (search) also voiced caution about the prospects of catching Al Qaeda leader Usama bin Laden, saying it is "too early to tell" if a new U.S. strategy aimed at winning the trust of Afghans will yield crucial intelligence.

The U.S. military praised Pakistan (search) for a crackdown in its South Waziristan tribal region in March. Still, the operation failed to net any top Al Qaeda men and Islamabad has now offered an amnesty to foreign fighters who eschew terrorism and agree to live peacefully.

Barno said the U.S. military was watching closely how Pakistan deals with the militants, but said a "significant" number had to be "killed or captured."

"It's very important that the Pakistani military continue with their operations to go after the foreign fighters in particular, who in my view will not be reconciled," Barno said.

"We have some concerns that (the Pakistani operation) could go in the wrong directions," he said. Attacks on U.S. forces just across the border from Waziristan are frequent, and militants often retreat into the mountains toward the Pakistani region.

On Saturday, the Pakistani government pushed back by one week an April 30 deadline for foreign militants to surrender. Despite a threat of renewed military action, none have taken up the amnesty offer.

Pakistani army spokesman Maj. Gen. Shaukat Sultan insisted there was no rift with the United States.

"Pakistan is saying nothing different from what the U.S. commander is saying. We also say that the foreign elements in our tribal areas must surrender, otherwise they will be killed," Sultan said.

The two-week Pakistani military operation in South Waziristan left more than 120 people dead. But officials say hundreds of militants escaped.

The limited success of the offensive has persuaded the Pakistani government to opt for negotiations instead of force.

The U.S. military and U.S. Ambassador Zalmay Khalilzad back talks to bring former Afghan militants back into the political mainstream. But Barno said Pakistan was grappling with a terrorist network that is "very crafty and has great ability to ultimately not give anything up."

"We clearly still see significant elements of foreign fighters there — Arabs, Chechens, Uzbeks ... who are still using that area to advance their terrorist aims," Barno said.

Pakistani authorities have released 141 of the 163 Pakistani and foreign suspects captured in March, saying investigations proved them innocent.

Some of the foreigners in Waziristan are Afghan refugees. Others are Central Asian and Arab veterans of the anti-Soviet war in Afghanistan in the 1980s who settled in Pakistan.

The Pakistani interior minister has said that all but top Al Qaeda and Taliban leaders can seek amnesties.

Khalilzad said recently that many of those leaders are living in Pakistan and the border region, where bin Laden may also be holed up.

The U.S. military has backed off earlier forecasts that it will capture the Al Qaeda leader this year.

Efforts to win over Afghans in border provinces with regular meetings and millions in reconstruction aid have helped uncover roadside bombs and weapons caches, Barno said.

"It's too early to tell the overall effect on our high-value targets that we're looking for," he said. Building relationships with community leaders and local officials "ultimately will be the key to our success," he said.

Barno has boosted his forces to some 15,000 from 11,000 at the end of last year, but the extra muscle has failed to prevent a surge of violence in southern and eastern Afghanistan.

More than 300 people have died across the country so far this year, including almost 100 in the past month.

In the latest bloodshed, officials said four Afghan soldiers and two civilians were killed in two mine blasts Sunday in southern Zabul province.

Barno said militants were "particularly desperate" to block Afghanistan's path to democracy, which includes plans for national elections in September.