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.