The resignations of the interior minister and intelligence chief come as Afghanistan is trying to combat a strengthening insurgency in the south. On Monday, Taleban suicide bombers killed at least two people — one of them an American contractor — in an attack on a police training center in southern Kandahar city, the US Embassy said.

Meanwhile, three NATO soldiers were killed in the south on Monday — two from a bomb attack and one in a gunfight, NATO forces said in a statement. It did not provide nationalities. The resignations on Sunday surprised Western officials and could cause disruption within Afghanistan’s intelligence and security establishment at a critical juncture as the US and NATO escalate the war and the government formulates plans to negotiate peace with the insurgents.

The move is likely to fuel speculation over differences within the Karzai administration over its efforts to reconcile with the Taleban — including the possible release of hundreds of detained militant suspects.

The head of the National Directorate of Security, Amrullah Saleh, was a senior figure in the Northern Alliance that helped the US oust the Taleban regime in 2001. As a young man, Interior Minister Hanif Atmar served in Afghanistan’s Communist-era intelligence agency and fought mujahedeen opposed to the Soviet occupation.

Saleh and Atmar resigned Sunday to accept responsibility for lapses that allowed militants to launch an attack on a conference last week of some 1,500 dignitaries from across the country to discuss peace with the insurgents.

Rockets landed near the conference venue, but the attack was thwarted with no injuries to officials. Security officials have rarely faced punishment or resigned over previous major attacks in the capital and elsewhere.

Karzai left Afghanistan on Monday to attend an international conference in Spain, canceling a news conference he had scheduled earlier. His spokesman, Waheed Omer, insisted the security lapse at the conference was the only reason for the resignations.

“This could have been national chaos, a national crisis” if the attack had succeeded, Omer told reporters.

“Somebody had to take responsibility for this.” US Defense Secretary Robert Gates, speaking to reporters on his way to London, stepped gingerly in answering questions about the abrupt resignations of Saleh and Atmar, whom US officials had often singled out by name as examples of competent leadership in a government riven by corruption and patronage.

“It’s obviously an internal matter for the Afghans,” Gates said.

“I would just hope President Karzai will appoint in the place of those who have left people of equal caliber,” he said.

Saleh, an ethnic Tajik, had served as intelligence chief since 2004 and had a long-standing relationship with the CIA in the fight against the Taleban and Al-Qaeda.

British-educated Atmar, a former education minister, was first appointed interior minister in a 2008 Cabinet reshuffle aimed at rooting out high-level corruption in Karzai’s government. He was reappointed after Karzai’s re-election.

Karzai wants to offer incentives to low-level insurgents to lay down arms, and to enter talks with Taleban leaders.

Some critics worry Karzai may be willing to give up human rights and other advancements to strike a deal with the Taleban to end the conflict.

Washington is skeptical about opening talks with the Taleban until they have been weakened on the battlefield.

Richard Holbrooke, President Barrack Obama’s special envoy on Afghanistan, on Monday voiced cautious support for peace overtures.

“We will support Afghan-led reconciliation,” Holbrooke told reporters on the sidelines of a conference in Spain.

“The US and the international community are not interposing objections.” But militants who enter the talks must renounce Al-Qaeda and accept the Afghan Constitution, he said.

The resignations come as NATO tries to build up Afghanistan’s police and armed forces so it can hand over security and begin drawing down troops along Obama’s timeline starting in mid-2011. As part of those plans, US

commanders are planning a major NATO operation soon in the Taleban’s heartland of Kandahar.

The Taleban have struck in Kandahar with a slew of bombings and assassinations of people seen as government allies — a counteroffensive to the NATO push.

On Monday, at least three suicide bombers attacked a police training center in Kandahar city, exploding a car bomb at the gate then trying to storm through with guns blazing, officials said. Along with the two deaths announced by the Americans, three police officers were wounded, officials said. All three attackers were killed.

In Uruzgan province, meanwhile, a civilian car hit a roadside bomb early Monday, killing two men inside, said provincial Police Chief Juma Gul Himmat.