Beijing Olympics: Afghanistan, Pakistan's new anti-militant front

President Barack Obama has gathered sometimes mistrustful leaders of Pakistan and Afghanistan to forge a new anti-terror front, but Afghan deaths in US raids have cast a pall over their high-stakes summit.

Obama called Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari to Washington to cement his new plan to the neighbors, each facing an upsurge in violence, into a joint bid to crush the Taliban and Al-Qaeda.

"I'm pleased that these two men, elected leaders of Afghanistan and Pakistan, fully appreciate the seriousness of the threat that we face, and have reaffirmed their commitment to confronting it," Obama said Wednesday.

Secretary of State Hillary Clinton meanwhile said the talks, which will continue at cabinet level Thursday, showed "very promising early signs."

Zardari was seeking US military aid and political support, while Karzai hopes to purge Taliban havens in Pakistan, which are destabilizing his country.

The talks came amid fresh deadly clashes in Pakistan's Swat valley -- launched under US pressure -- in which the military said it had killed more than 80 militants in an upsurge of fighting.

The talks also coincided with fresh reports from Afghan police that US-led air strikes targeting insurgents had killed 100 people, most of them civilians, in one of the deadliest battles in nearly eight years.

The US military opened an investigation into the operation overnight Monday in the remote western province of Farah, and Karzai ordered his government to probe reports of high civilian casualties.

"I made it clear that the United States will work with our Afghan and international partners to make every effort to avoid civilian casualties," Obama said after talks with the two presidents.

US national security advisor James Jones said Obama opened his meeting with Karzai by expressing regret over civilian deaths.

"It was clear that President Karzai was moved by that ... and he thanked the preside nt for starting off the meeting with that expression of condolence," Jones said.

Clinton, who met the two leaders before Obama, also added her condolences.

"We deeply regret it. We don't know all of the circumstances or causes. And there will be a joint investigation."

The secretary of state also expressed hope the Washington talks would renew the battered joint effort to combat extremism.

"I am very optimistic that this process is making a difference," Clinton said, though she added she was aware that a few meetings could not on their own solve the myriad of issues confronting the two nations.

"Both presidents spoke very movingly about the threat and dangers of terrorism," Clinton said, adding she was "extremely impressed" by the candor of the two leaders and their delegations.

Clinton said on Wednesday she was impressed with recent efforts by Pakistan to fight extremism, following her recent harsh criticisms of its conduct.

He said Pakistan bore a "huge burden" in fighting both the Taliban and Al-Qaeda, but added "we are up to the challenge because we are the democracy and democracy is the only cure to this challenge."

Marai