Afghanistan's Hamid Karzai and Pakistan's Asif Ali Zardari came to Washington after heavy criticism of their efforts to combat a Taliban resurgence in their countries.

After their talks Obama said both men "fully appreciate the seriousness of the threat" posed by al Qaeda and their allies.

"The road ahead will be difficult. There will be more violence and there will be setbacks," Obama said, with Karzai and Zardari at his side in the Grand Foyer of the White House.

"But let me be clear -- the United States has not only made a lasting commitment to defeat al Qaeda but also to support the democratically elected sovereign governments of both Pakistan and Afghanistan. That commitment will not waiver and that support will be sustained."

The Obama administration has sharply criticized both Karzai and Zardari in the past, questioning their commitment and capability to tackle the threat from al Qaeda and the Taliban.

Civilian casualties caused by US military operations in both countries, including drone strikes inside Pakistan, have infuriated many Afghans and Pakistanis and made it harder for both countries' leaders to cooperate with the United States.

Military action earlier this week is believed to have caused dozens of civilian deaths with regional lawmakers and the International Committee of the Red Cross reporting that as many as 100 people may have died in an operation in the Bala Boluk district of Farah province in western Afghanistan.

"We deeply, deeply regret that loss," US Secretary of State Hillary Clinton said of the civilian deaths before meeting Karzai and Zardari.

Later she called that meeting "in some ways a breakthrough" and said she was "very optimistic" the process was making a difference.

Obama announced a new approach to the fight against al Qaeda in both countries in late March, offering more aid but also more than 20,000 extra troops to Afghanistan this year.

Pakistan's Zardari, who has been under fierce criticism for his response to Taliban militants who have made inroads in the Swat and Buner valleys this year, pleaded for support for his fledgling democracy.

"My democracy needs attention and needs nurturing," Zardari said. "Pakistani democracy will deliver, the terrorists will be defeated by our joint struggle. Me, my friend President Karzai and the United States ... will stand shoulder to shoulder with the world to fight this cancer and this threat."

The Pakistan government recently launched a major military offensive following pressure from the US to stop the advance of extremists in the region. The Islamist Taliban says the offensive violates a peace deal it made with Islamabad, which allowed the introduction of Shariah law in the Swat valley in exchange for a ceasefire.

On Wednesday, Pakistani security forces attacked Taliban fighters in the Swat valley, killing at least 64 of them after the United States called on the government to show its commitment to fighting militancy.

But the growing Taliban influence in north-western Pakistan has raised international concerns over the stability of the nuclear-armed country.

Meanwhile in Berlin, Pakistani Defence Minister Chaudry Ahmad Mukhtar has met with his German counterpart Franz Josef Jung and asked for international help. Mukhtar said Pakistan did not have the capacity to fight what he described as a "guerrilla war" in the north-west of the country.