KABUL, Afghanistan -- An Afghan official accused Pakistan's premier spy agency on Wednesday of organizing a recent assassination attempt on Afghan President Hamid Karzai in the most serious in a string of allegations against Pakistan.

The charge bodes ill for U.S. efforts to get Pakistan's new government to work with Karzai's embattled administration to counter Islamic militants on their common border.

Karzai escaped unharmed when assailants fired guns and mortars toward the president, senior officials and foreign diplomats during a military parade in downtown Kabul on April 27. Three Afghans were killed.

Since then, Karzai has ramped up his criticism of Pakistan, whom Afghan officials have long suspected of secretly aiding the insurgents. Karzai even threatened to send troops into Pakistan to eliminate Taliban leaders earlier this month.

Afghan intelligence chief Amrullah Saleh alleged last month that suspects involved in the attempt on Karzai had exchanged cell phone text messages with people in Pakistan's lawless tribal regions and the city of Peshawar.

Presidential spokesman Humayun Hamidzada said Tuesday that "the hand of one foreign intelligence agency was clearly involved."

Saleh's spokesman, Saeed Ansari, went further Wednesday, claiming Afghan intelligence could prove Pakistan's Inter-Services Intelligence agency, or ISI, was involved.

"The evidence and documents as well as the confessions of people arrested by the intelligence service shows that the main organizer of the terrorist acts during the 16th anniversary of the mujahedeen victory was the intelligence service of Pakistan and its allies," Ansari said.

Pressed at a news conference for details, Ansari said one piece of evidence was a secret code used during phone conversations between militants. He released a list of phone numbers prefixed with Pakistan's country code, 92, which he said they had used.

However, he said couldn't reveal all the reasons the Afghan agency was certain the ISI was involved because it could compromise its operations.

"We are sure and confident" of an ISI connection, he said.

Ansari also said that militants involved in a deadly attack on Kabul's Serena Hotel in January had links with the ISI.

A spokesman for the Pakistan army, which controls the ISI, was not immediately available for comment. The Foreign Ministry said it would only respond after viewing a transcript of Ansari's remarks.

Pakistani leaders acknowledge that Taliban and al-Qaida militants find refuge in its lawless tribal belt. However, they angrily reject suggestions that its security agencies are colluding with militants.

"Pakistan will not allow its territory to be used against other countries, especially Afghanistan," Prime Minister Yousuf Raza Gilani said Wednesday.

But in a message to both Afghanistan and the United States, he added: "Under no circumstances will foreign troops be allowed to operate inside Pakistan."

Cross-border relations are already strained in the wake of a U.S. air strike which Pakistan says killed 11 of its soldiers last month.

Pakistani officials complain that they are being unfairly blamed for the failure of the Karzai government to stabilize Afghanistan, despite billions of dollars in international aid and the presence of 60,000 foreign troops.

In the latest fighting, the U.S.-led coalition said Afghan police called for help Tuesday night when insurgents armed with rockets and guns attacked government offices in the Sarobi and Gomal districts of Paktika province.

"When coalition air support arrived, the 22 militants who attacked the district centres were positively identified and killed," a coalition statement said.

Ghamai Khan Mohammadyar, spokesman for the provincial governor, said police counted the bodies of 22 "enemy fighters" on the battlefield. He said no Afghan forces were killed and claimed that the surviving militants had fled toward the Pakistani border.

The coalition said one of its troops died and three others were wounded when a bomb hit a vehicle on a combat operation in Helmand province.

One NATO soldier also died in Helmand when an explosion hit a patrol in Nahri Sarraj district on Tuesday, the alliance said.

More than 2,000 people have died in insurgency-related violence this year, according to an Associated Press tally.

Many are militants slaughtered by U.S. warplanes responding to skirmishes on the ground. A total of 111 foreign troops have also died, including 36 this month -- a faster monthly pace of attrition than in Iraq.