Officials say three Pakistanis' alleged involvement in a plot to assassinate the US ambassador here is evidence that Islamabad is not doing enough to stop terrorism, or is complicit in it.
The rift is bad news for Washington -- which counts both countries as essential allies in the war on terrorism.
Afghan officials have charged for weeks that Taliban and al-Qaeda agents were slipping in from Pakistan -- and that they were behind two deadly suicide bombings, the kidnapping and killing of Afghan security forces, and several major confrontations with the US-led coalition.
'Al Qaeda regrouping in Afghanistan'
Defense Minister Rahim Wardak told The Associated Press last week that rebels were receiving support from "regional powers" rattled by Afghanistan's request for a long-term US and NATO presence.
"There is no doubt that there are countries in this region that have their own designs, and have had from long ago, and they are always trying to exploit the vacuums that have been created here," Wardak said.
He didn't single out any country, but strongly hinted he was referring at least partly to Pakistan.
Officials here say Islamabad is eager to resume its traditional role as regional power broker, and feels threatened by Kabul's warm relations with Pakistan's archrival, India.
Pakistan vehemently denies any involvement in terrorism, saying it has done more than any other country in the fight against al-Qaeda. About 70,000 Pakistani troops have fanned out along the border, and Islamabad boasts turning over 700 al-Qaeda suspects to the United States.
In Islamabad, Pakistani Information Minister Sheikh Rashid Ahmed expressed outrage at the Afghan statements.
"Let us make it clear that Pakistan as a state is not involved in any unlawful activity on the Afghan soil, and such claims and allegations from the Afghan side must stop," he said. "No Taliban leaders are hiding here."
Ahmed said his government supports Afghan President Hamid Karzai, despite domestic sentiment against the policy.
"We have paid a political price by supporting him, but this support will continue," he said.
Pakistani political analyst Talat Masood said that both countries have a history of blaming each other for their woes, and that the public war of words was a dangerous distraction.
"The more they blame each other publicly, the more their relations are strained and the cooperation gets worse, to the advantage of the militants," he said.
Washington has been forced to walk a tightrope to try not to offend either side.
President Bush phoned Pakistani President Gen. Pervez Musharraf on Tuesday, according to White House press secretary Scott McClellan. Pakistani and Afghan officials say Musharraf and Karzai also spoke Tuesday.
US military spokesman Col. James Yonts said Monday that foreign militants, backed by networks channeling them money and arms, had come into Afghanistan to try to subvert parliamentary elections slated for September.
He said that for "operational security reasons" he could not identify the networks, nor say who supported them.
Since March, hundreds of people -- including at least 29 US troops -- have been killed in a surge of violence across the south and east. This month, suicide bombers killed 20 people in a crowded mosque and wounded four US troops in a convoy. Afghan calls for Pakistan's help in stopping the violence have grown more strident. After the assassination plot against US Ambassador Zalmay Khalilzad was revealed, Kabul officials took the gloves off.
"Some senior members of the Taliban, including some who are involved in killings and are considered terrorists, are in Pakistan," presidential spokesman Jawed Ludin said Tuesday at a Kabul news conference.
Violence is worst near the border, Ludin said.
Pakistan, Afghanistan decide to bury past
"Our people are dying, our schools are getting burned, our mosques are getting blown up and our clergy are getting assassinated," he said. "Some provinces of the country, especially in regions that are close to Pakistani soil, are insecure in many ways."
A senior official close to Karzai scoffed at suggestions that rogue elements of Pakistan's intelligence service, Inter Services Intelligence, or ISI, might be supporting militants without Musharraf's knowledge.
The official, who spoke on condition of anonymity due to the issue's sensitivity, said Pakistan had played a major role in keeping Afghanistan's October presidential election safe, sealing the border and going after terrorists.
But that cooperation has ended, and Afghan officials say they now think Pakistan was less than sincere.
Ludin said he was not giving up on relations improving, but he offered only cautious optimism.
"Neither Afghanistan nor the international coalition against terrorism will achieve success if we don't get the level of cooperation from Pakistan that we have had in the past," he said.
"We are hopeful and we are confident that that (cooperation) will be forthcoming ... but at the moment as far as the situation goes, we still have more work to do."
(Munir Ahmad in Islamabad contributed to this report.)
More reports from: Pakistan | Afghanistan