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KABUL, Afghanistan - Afghan officials blamed a brazen series of weekend attacks on the Haqqani militant network, saying Monday that fighters captured in the assault claimed they were affiliated with the insurgent faction tied to the Taliban and al Qaeda.

The 18-hour offensive left 36 insurgents and 11 others dead and was the largest in Kabul since insurgents fired on the U.S. Embassy and NATO headquarters last September. That attack also was also blamed on the Haqqani network, which commands the loyalties of an estimated 10,000 fighters and is considered one of the most lethal threats to NATO in Afghanistan.

Afghan Interior Minister Besmillah Mohammadi said one militant arrested during Sunday's assault on Kabul and three other cities confessed that he was loyal to the Haqqanis. An Afghan intelligence official said three other insurgents detained for allegedly plotting to assassinate one of the nation's two vice-presidents also said they were members of the Haqqani network.

And officials in two provinces said they too suspected that attacks in their cities were the work of the Haqqanis.

The Haqqanis, led by Jalaluddin Haqqani and his son Sirajuddin, operate primarily in provinces along Afghanistan's eastern border with Pakistan. NATO spokesman Carsten Jacobson once described the group as a "family clan, a criminal patronage network and a terrorist organization."

Former Joint Chiefs of Staff Chairman Adm. Mike Mullen said in October 2011 the Haqqanis acts as a "veritable arm" of the Pakistani intelligence agency -- an accusation Islamabad denied. Mullen accused the network of staging the Sept. 13 attack on the U.S. Embassy and NATO headquarters, as well as a truck bombing that wounded 77 American soldiers in Wardak province.

During the series of attacks that continued into Monday morning, eight policemen and three civilians were killed along with 36 insurgents, Mohammadi said.

"One of the terrorists who has been arrested in Jalalabad has confessed that they were trained and equipped outside of our borders," Mohammadi told a news conference. "He has confessed that they were in one of the branches of the Haqqani network. We have his confession."

Lutfullah Mashal, a spokesman for the Afghan intelligence service, said two suicide bombers and another insurgent arrested on Sunday on the west side of the city had confessed to being members of the Haqqani network. He said the three are suspected of plotting to kill Vice-President Karim Khalili.

Apart from Kabul, the eastern capitals of Paktia, Logar and Nangarhar provinces also came under attack Sunday as suicide bombers tried to storm a NATO base, an airport and police installations there.

Abdul Rahman Mangal, deputy governor of Paktia province, said local intelligence agents blamed Haqqani for the attack in Gardez, the provincial capital.

"There's nobody else who could have done it," Mangal said. "Our intelligence department told us that the Haqqani network is behind this attack. The Haqqanis are close to Miram Shah (Pakistan) and from there, they can easily come to Paktia province."

Gen. Ghulam Sakhi Roogh Lawanay, police chief in Logar province, said investigators also were convinced that the Haqqani network orchestrated the attack in Logar.

"We found mobile phones and documents and the telephone numbers showed that there was contact between a remote area in Afghanistan and the Pakistani side of the border," he said. "The Haqqani network was behind the attack."

Still Afghan officials may have political motivations for pointing the finger at Haqqani.

Afghan and U.S. officials are trying to coax the Taliban fighters -- who are not as closely linked with al-Qaida as the Haqqanis -- to negotiate a political resolution to the 10-year-old war.

If the Haqqani faction is behind the attacks, it could be easier to sell the idea of making peace with the Taliban to skeptics.

President Hamid Karzai met at the presidential palace with a delegation from the third major insurgent faction in Afghanistan known as Hizb-i-Islami. The radical Islamist militia has thousands of fighters and followers across the north and east. Its leader, powerful warlord Gulbuddin Hekmatyar, is a former Afghan prime minister and one-time U.S. ally who is now listed as a terrorist by Washington. The delegation is led by Hekmatyar's son-in-law, Ghairat Baheer.

Karzai said the attacks were an "intelligence failure by us and especially NATO" that allowed the militants to enter Kabul and other targeted cities, and called for a full investigation. However he praised the Afghan security forces' response to the attacks.

Though the death toll was much lower than in other attacks, the dramatic assault on multiple targets showed that militants are far from beaten and can still penetrate Afghan security -- even in the heart of the capital -- after 10 years of war. The attacks also underscored the security challenge facing government forces as U.S. and NATO troops draw down and prepare to leave by the end of 2014.

Taliban spokesman Zabiullah Mujahid said the attacks had been planned for two months to show the insurgency's potency after NATO officials called the Taliban weak. He told The Associated Press that they did not mark the start of the insurgents' spring offensive, which would begin shortly.

"It is a message for the spring offensive but it has not yet started," Mujahid said.

The attacks on the Afghan capital ended Monday morning when insurgents who were holed up overnight in two buildings were overcome by heavy gunfire from Afghan-led forces and pre-dawn air assaults from U.S.-led coalition helicopters.

Rocket-propelled grenades were fired one after another into a building in the centre of the city, from where the insurgents launched one of their attacks on Sunday. The building, which is under construction, overlooks the presidential palace, Western embassies and government ministries.

The U.S., German and British embassies and some coalition and Afghan government buildings took direct and indirect fire, according to Lt. Col. Jimmie Cummings, a spokesman for the U.S.-led coalition.

"A Haqqani connection is a possibility, but still too early to determine for sure," said Cummings, the NATO spokesman. "We will look strongly at that."