The U.S. ambassador to Afghanistan said Thursday that there is "no question" that the Haqqani network was behind this week's brazen attacks on Kabul and other eastern cities and said Pakistan needs to do more to clamp down on the group's safe havens.

The comments were the strongest yet from a U.S. official blaming the attacks on the Haqqani network, an insurgent group based in neighboring Pakistan that is allied with the Taliban and Al Qaeda and has been described by American officials as the most dangerous in the Afghan war.

Sunday's coordinated assault included near-simultaneous attacks in the three parts of the capital and three other eastern cities. Eight policemen and three civilians were killed in 18 hours of fighting, along with 36 militants, according to Afghan officials.

"There is no question in our mind that the Haqqanis were responsible for these attacks," U.S. Ambassador Ryan Crocker told reporters in Kabul. "We know where their leadership lives and we know where these plans are made. They're not made in Afghanistan. They're made in Miram Shah, which is in North Waziristan, which is in Pakistan."

"We are pressing the Pakistanis very hard on this," Crocker said, echoing statements from Secretary of State Hillary Rodham Clinton earlier this week. Clinton said on Tuesday that there were indications that the Haqqanis were involved.

Washington has long demanded that Pakistan target the Haqqani network, which is based in northwest Pakistan and allegedly has ties to the country's spy agency, the Inter-Services Intelligence. Islamabad has firmly rejected the accusations, and has condemned Sunday's attack.

The Haqqanis have become one of the most difficult Taliban factions to figure out how to deal with in a plan to bring the insurgents to the negotiating table with the Afghan government.

They are seen as more ideologically tied to Al Qaeda than some of the other militant groups, and they have been particularly adept at sophisticated strikes like the one this week. The group was also blamed for a similarly audacious attack in September in which militants took over a half-constructed high-rise in the capital and opened fire on the nearby NATO headquarters and U.S. Embassy with heavy weapons.

Crocker described the Haqqanis as "a group of killers, pure and simple," and said they may be a faction of the Afghan insurgency that is irreconcilable. He argued that the solution to the Haqqani problem would be found in Pakistan.

"I don't think it's a question of going to war with Pakistan to solve this problem. It's getting the Pakistanis to take the necessary action themselves to eliminate a threat not only to Afghanistan and the United States but also to Pakistan," Crocker said.