KABUL, Afghanistan — In a sign of worsening relations between two key U.S. allies, Afghanistan's government on Tuesday accused Pakistan of refusing to cooperate in the investigation into last month's assassination of the head of the Afghan peace commission — a plot the Afghans say was hatched in Pakistan.

But Pakistani authorities immediately denied they'd refused to cooperate. "We completely reject this, this is not true," Foreign Ministry spokeswoman Tehmina Janjua told reporters in Islamabad.

The competing claims underscore the tension that surrounds the Sept. 20 slaying of Burrhanuddin Rabbani, a former Afghan president who was killed in his home when a suicide bomber he thought was a Taliban emissary interested in peace talks detonated explosives hidden in his turban.

Last week, Afghan authorities said the plot had been hatched in Quetta, Pakistan, where the Taliban's leadership is believed to keep its headquarters, and that Pakistan's spy agency, the Inter Services Intelligence Directorate, had played a role in the bombing.

On Tuesday, in an effort to bolster claims of a Pakistan link to the killing, the deputy chief of Afghanistan's intelligence agency identified the bomber as a Pakistani citizen named Esmattullah, 24, from the border town of Chaman.

Mohammed Yasin Zia, the deputy chief of Afghanistan's National Directorate of Security, said Pakistan had conveyed its refusal to cooperate in a letter from its embassy in Kabul. He said the Pakistanis cited news coverage of "sensitive information" as the reason the Islamabad government would not cooperate.

"We provided them with all necessary evidence to link the Quetta Shura to the assassination, but as usual the Pakistanis are making excuses," Zia said. The Quetta Shura is the council of Taliban leaders who meet in Quetta.

Mohammad Sadiq, Pakistan's ambassador in Kabul, told the Reuters news agency that he had neither refused to help nor complained about media leaks.

"There is a clear misunderstanding," he said in a phone interview, according to Reuters. "We sent a letter to the commission set up to investigate Professor Rabbani's killing and reaffirmed our cooperation. We never said that we are not cooperating."

In Islamabad, spokeswoman Janjua said Prime Minister Yousuf Raza Gilani had offered cooperation in the investigation when he visited Kabul Sept. 22 to express his condolences over the assassination.

"Pakistan stands by its commitment to help Afghanistan in investigating Mr. Rabbani's assassination," she said.

Last week, Afghan officials said they'd given the Pakistan Embassy in Kabul the names, addresses and phone numbers of the alleged plotters and asked for Pakistan's cooperation. They said they'd even provided a layout of the house in Pakistan where the assassination was plotted.

It was unclear what evidence links Pakistan's ISI to Rabbani's death. The charge that the ISI was behind the killing was first leveled last week by Afghanistan's interior minister, Bismillah Khan Mohammedi, who made the charge before the lower house of parliament.

But it tracked a similar claim by the former chairman of the U.S. Joint Chiefs of Staff, Adm. Mike Mullen, that the ISI had been involved in an attack in Kabul on the U.S. Embassy.

On Monday, President Hamid Karzai blasted Pakistan in a nationally televised speech, accusing it of not being a partner for peace.

"Pakistan has pursued a double game toward Afghanistan, and using terrorism as a means continues," Karzai said. Karzai spoke before departing for India to sign a strategic agreement with that country, Pakistan's archenemy.

Afghanistan said it would suspend efforts to negotiate an end to violence in the country with the Taliban and would postpone a meeting of the joint Pakistan-Afghanistan peace and reconciliation commission planned for later this month. Pakistan's prime minister had planned to attend the meeting.

(Zohori is a McClatchy special correspondent. Saeed Shah in Islamabad, Pakistan, contributed to this report.)

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