Washington  - The leaders of the United States, Pakistan and Afghanistan vowed Wednesday to work together to confront a growing and common enemy in the Taliban militants and al-Qaeda terrorists operating along the restive border region.

"The security of Pakistan, Afghanistan and the United States are linked," said Obama, who held a three-way meeting with Afghan President Hamid Karzai and Pakistani President Asif Ali Zardari at the White House as well as separate talks with both leaders.

Karzai and Zardari promised closer cooperation amid an upsurge in fighting against militants in both countries.

The Pakistani military launched a major offensive this week in its north-western provinces against Taliban forces, after a fragile and controversial peace deal between the two sides collapsed.

Foreign ministers from the neighbouring countries signed an agreement to start talks on a transit deal to be completed by the end of the year, which could help loosen border restrictions and improve infrastucture for trade.

The three-way summit included government officials from agencies including agriculture, justice and intelligence.

"Our threat is common and our responsibilities should be shared," Zardari said earlier Wednesday, calling Pakistan's insurgency a "cancer (that) needs to be done away with."

Karzai characterized Afghanistan and its neighbour as "conjoined twins" with a long history and a share in both the successes and struggles against the militants.

As the new Obama administration sends 17,000 extra troops to Afghanistan, the president promised to make "every effort" to avoid civilian casualties, after military action earlier this week is believed to have caused dozens of civilian deaths.

Regional lawmakers and the International Committee of the Red Cross said that as many as 100 people may have died in an operation in the Bala Boluk district of Farah province in western Afghanistan, though it remained unclear whether it was the result of US or Afghan military actions. Civilian casualties from US airstrikes have long strained US-Afghan relations.

US Secretary of State Hillary Rodham Clinton, who met earlier Wednesday with Karzai and Zardari, said: "We deeply, deeply regret that loss," and promised a joint investigation with the Afghan government.

Clinton said the summit was "in some ways a breakthrough meeting" that helped build trust between the two sides and brought together many groups for the first time.

The talks came as Pakistan's military began a major assault against Taliban forces in two north-western districts, killing more than 60 militants Wednesday and losing four soldiers.

Taliban strength in north-western Pakistan - stretching as close as 100 kilometres from the capital Islamabad - has prompted international concerns over the stability of nuclear-armed Pakistan's government.

Obama has identified the regional conflict as his top national security priority, promising increased non-military support to both countries as well as the additional troop deployments.

Obama said Karzai and Zardari "fully appreciate the seriousness of the threat that we face and have reaffirmed their commitment to confronting it." He pledged 400 million dollars in immediate military aid to help Pakistan's latest offensive.

"Pakistani democracy will deliver," said Zardari, who took office as president eight months ago. "The terrorists will be defeated by our joint struggle."