Washington - US Secretary of State Hillary Rodham Clinton apologized Wednesday for civilian casualties after a US airstrike this week may have killed more than 100 people in western Afghanistan.

Speaking after a meeting with the leaders of Afghanistan and Pakistan, Clinton said "we deeply, deeply regret that loss" and promised a joint investigation with the Afghan government.

Afghan lawmakers and the International Red Cross Committee said dozens of villagers lost their lives in an operation in the Bala Boluk district of the western Farah province. Civilian casualties caused by US strikes have been an ongoing source of tension between the US and Afghan leaders.

US President Barack Obama was to hold a three-way meeting at the White House later Wednesday with Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari.

Both regional leaders promised to work together to defeat common threats from the Taliban in their countries, while their foreign ministers agreed to start talks on a transit agreement to be completed by the end of the year.

Clinton said bringing peace to the region required an "all government effort" that went well beyond military support, amid an uptick in fighting against the Taliban in both Afghanistan and Pakistan.

The Taliban has been growing in strength along the border region between both countries and has taken control of growing area of Pakistan in recent weeks.

"Our threat is common and our responsibilities should be shared," Zardari said, calling the insurgency in his country a "cancer (that) needs to be done away with."

"Pakistani democracy will deliver. The terrorists will be defeated by our joint struggle," Zardari said.

Karzai called the two countries "conjoined twins" that had a share in both the successes and struggles against the insurgents.

Obama has identified the regional conflict as his top national security priority, ordering an extra 17,000 US troops to Afghanistan to confront the Taliban and al-Qaeda, as well as increased non- military support to both countries.

The possibility of an extremist takeover in nuclear-armed Pakistan has been of increased concern to the United States and the international community. But Admiral Michael Mullen, chairman of the US Joint Chiefs of Staff, told reporters Monday he was "comfortable" the nuclear arsenal was safe.