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Kabul, Afghanistan (CNN) -- The Afghan government on Wednesday blamed an al Qaeda-linked militant group based in Pakistan for the siege on a Kabul hotel that left 12 victims and all nine attackers dead.

The attackers, all of whom were prepared to carry out suicide bombings, were with the Haqqani network, a group of terrorists loyal to the warlord Siraq Haqqani, a spokesman for the Afghanistan Interior Ministry said.

A Kabul-based official with direct access to security information also told CNN it is believed the attack was orchestrated by the Haqqani network.

Falak Merzahi, a spokesman for the Afghanistan Interior Ministry, said the attackers came into Afghanistan from Pakistan. They entered the Hotel Inter-Continental late Tuesday night by avoiding the main entrance and attacking a smaller one on the other side of the hotel, which was guarded by two Afghan police. The attackers killed the two officers and stormed the hotel, Merzahi said.

Six of the attackers ended up detonating their explosives; three were shot and killed on the roof of the hotel, Afghan officials said.

Although a NATO helicopter carrying International Security Assistance Force snipers flew to the scene and fired at the attackers, Merzahi said it was Afghan National Army soldiers who ultimately killed the three gunmen on the roof.

An ISAF official said ISAF forces stopped firing on the roof when Afghan soldiers arrived.

The 12 others killed included the two police officers, nine Afghan civilians, and one foreign national, Merzahi said.

Spain's news agency EFE reported it was a Spanish citizen, 48-year-old Antonio Planas, who was killed. Citing a family source, EFE said Planas, a pilot, leaves behind a wife and a daughter.

Two Special Operations Forces from New Zealand "received moderate injuries" in responding to the attack, the New Zealand military said.

A Taliban spokesman claimed the Taliban was behind the attack. "One of the suicide attackers told us on the phone that they are in the lobby and chasing guests into their rooms by smashing the doors of the rooms," Zabiullah Mujahid told CNN in an e-mail, as the incident was unfolding.

The Haqqani network, based in Pakistan's North Waziristan frontier, is believed to be closely allied to the Taliban.

The network has staged many spectacular attacks in Kabul in recent years and has the longstanding goal of trying to destabilize the Karzai government. "Confidence is high" in the information that the Haqqanis were behind the attack, the Kabul-based official said.

The attackers wore suicide belts, the official said.

While NATO helped Afghan police and military end the attack, Afghan President Hamid Karzai said it will not interrupt the planned handover of power from international forces to Afghan troops.

ISAF sent a similar message, praising "the rapid response by Afghan security forces who cleared the building and secured the situation."

"This attack will do nothing to prevent the security transition process from moving forward," said Rear Adm. Vic Beck, ISAF spokesman. U.S. President Barack Obama's administration has said U.S. troops will start withdrawing from Afghanistan in July, and that a military handover should be completed in 2014.

"Our thoughts and prayers are with the families of the dead and injured," Beck said in a statement. He added, "Even though insurgents have declared their intention to avoid civilian casualties, this attack put Afghan lives at risk and demonstrates their complete disregard for the Afghan people."

Karzai condemned the "terrorists" who "have no mercy on killings of civilians."

The attack came on the eve of a news conference that was scheduled to take place at the hotel Wednesday to discuss the planned transition of security from international to Afghan forces that U.S. President Barack Obama announced last week.

The news conference was canceled, and the hotel remained closed Wednesday.

Afghan authorities said they believe the attackers crept up through woods near the hotel to evade police checkpoints on the main road.

One attacker detonated a suicide vest in the lobby, causing chaos, officials said. At least five accomplices then stormed upstairs, ultimately making it to the roof.

Afghan commandos were among those who arrived shortly after, officials said.

About five hours later, a NATO helicopter carrying snipers fired on the roof. A U.S. Blackhawk helicopter carried the ISAF snipers, two coalition military officials told CNN. While ISAF has not given the nationality of the snipers, two coalition military officials told CNN they were not from the United States. Afghan troops also made it to the roof, officials said.

Erin Cunningham, a journalist for The Daily in Kabul, said that during the siege, rocket-propelled grenades were launched from the roof of the hotel toward the first vice president's house. A few moments later, the hotel was rocked by three explosions, one of which knocked her off her feet, Cunningham said. U.S. forces were on the scene, she added.

At about 3 a.m., ISAF said, Afghan security forces had cleared the roof and were clearing the rest of the hotel.

"The last suicide attacker was killed at around 7 a.m. during the search operation," Interior Ministry spokesman Siddiq Siddiqi said.

There were no indications that U.S. military or diplomatic personnel were staying at the hotel, U.S. officials told CNN.

While members of the Afghan National Security Forces were on the scene, the city police took the lead, ISAF Maj. Jason Waggoner said in a statement. Waggoner said ISAF forces provided "some limited assistance."

The United States condemned the attack, with State Department spokeswoman Victoria Nuland saying it "once again demonstrates the terrorists' complete disregard for human life."

The hotel was developed by the InterContinental Hotels Group and opened in 1969. But the hotel has had no association with the group since the Soviet invasion in 1979. It continues to use the name and logo without connection to the parent company.

The attack came a day after representatives from more than 50 counties attended a two-day International Contact Group conference in Kabul, according to Janan Mosazai, spokesman for the minister of Foreign Affairs.

He said "the role of neighboring countries in Afghan peace efforts, security handover, peace talks and strategic partnership between Afghanistan and the international community beyond 2014 were discussed in this conference."

The incident also came on the same day that Lt. Gen. William B. Caldwell announced that NATO and other members of the international community involved in Afghanistan have decided to increase the number of security forces in the Afghan National Army and the Afghan National Police to 352,000.

The current number of Afghan National Army and Afghan National Police is about 300,000, the commander of the NATO training mission in Afghanistan and commanding general of the Combined Security Transition Command told the Atlanta Press Club.

The increased number will be sufficient to give the Afghans security without coalition forces having to do it, he said.

Tuesday's attack stirred memories of the January 2008 attack at the Serena Hotel in Kabul, which killed seven people. The Taliban also claimed responsibility for that attack.

CNN's Reza Sayah in Islamabad, Pakistan; Matiullah Mati in Kabul; Barbara Starr, Larry Shaughnessy and Elise Labott in Washington; and journalists Jerome Starkey, Fazel Reshad and Jonathan Boone in Kabul contributed to this report.