WASHINGTON (AFP) - A key US senator on Thursday pledged quick action on a giant aid package to stabilize Pakistan as a committee cleared an early one billion dollars to support the insurgency-hit US ally.

Pakistani President Asif Ali Zardari and Afghan President Hamid Karzai warned of urgent challenges in a luncheon with members of the US Senate a day after a three-way summit with President Barack Obama.

Obama has pledged a new focus on fighting Taliban and Al-Qaeda extremists in the region. His special envoy, Richard Holbrooke, said the three nations would hold another meeting after Afghanistan's presidential election in August.

"Obviously, it is urgent," Kerry told reporters afterward.

"Senators on both sides - Republicans and Democrats - are committed to move this as rapidly as possible," Kerry said.

Shortly after the meeting, a key committee of the House of Representatives cleared an initial one billion US dollars for Pakistan as part of an emergency spending package to fund the wars in Iraq and Afghanistan.

The money approved by the House Appropriations Committee includes 400 million dollars to help build up Pakistani forces' counterinsurgency abilities amid a major offensive in the troubled Swat Valley.

It also includes another 600 million US dollars in economic development aid to improve education and boost democratic reforms in Pakistan, where Zardari's civilian government last year ended nearly a decade of military rule.

But the longer-term aid package to Pakistan is the subject of hot debate between the two chambers of Congress.

Some House members are pushing to attach conditions to the US aid, saying Pakistan must show accountability. Leading senators support a set of benchmarks to gauge progress but not trigger an automatic cut-off in aid.

Zardari's government has bristled at talk of explicit conditions, saying they were politically unfeasible considering that many Pakistanis were already suspicious of US intentions.

Kerry hinted of tough talks behind closed doors, calling the luncheon an "unprecedented frank exchange."

Lugar said senators pressed Pakistan on purported links to the Taliban by rogue intelligence elements in the nuclear-armed US ally.

But Kerry said the senators were not dictating "what the United States wants Pakistan or Afghanistan to do."

"We were here to listen to the presidents and learn what they believe is needed for their country and what the United States can do to help. This is their fight," he said.

The meeting came as Zardari's government urged the nation to unite and mounted air and ground strikes against Taliban extremists who have waged a two-year insurgency in the Swat Valley.

The offensive came three months after Zardari entered into a truce - harshly criticized in Washington - that put three million people in the region under strict Islamic law.

Zardari pledged to carry on the offensive "until life in Swat comes back to normalcy."

Karzai said he wanted action by the United States to end civilian deaths in US military operations, which he said "causes pain to Afghans."

"It's something that we want to have addressed very, very much and in a manner that eventually - or, rather, sooner - ends casualties," Karzai said.

But the Afghan leader said he was "happy" with the US response so far, noting that Obama and Secretary of State Hillary Clinton both brought up the issue with him during their meetings.

The western Afghan town of Farah has been rocked by sometimes violent protests against the reported killings of up to 70 civilians during US-led air strikes and fighting against insurgents.