The U.S. denied there was any incursion but the reports threatened new rifts between Washington and its key anti-terror ally days after a truck bombat a luxury hotel in Islamabad.
Nuclear-armed Pakistan is under growing U.S. pressure to act against al Qaeda and Taliban insurgents sheltering in its border region and blamed for rising attacks on coalition troops in Afghanistan as well as suicide bombings in Pakistan.
Afghanistan wants to set up a joint military force that would have the power to operate on both sides of the border with Pakistan, where militants have found safe haven.
Afghan Defense Minister Abdul Rahim Wardaksaid said Monday that he has proposed creating a joint force to include coalition forces operating on Afghan territory and Pakistanis operating on their side.
"A terrorist does not recognize any boundaries," Wardak said. "So to fight them I think we have to eventually come up with some arrangement together with our neighbor, Pakistan - that we should have a combined and joint task force of coalition, Afghan and Pakistanis to be able to operate on both sides of the borders regardless of which side."
U.S. officials believe that al Qaeda's leaders, including Osama bin Laden, are hiding somewhere along the border.
Late Monday, Dubai-based TV channel Al-Arabiya said it had received a tape from a shadowy group calling itself "Fedayeen Al-Islam" - Arabic for "Islam commandos" - claiming responsibility for the bombing and calling on Pakistan to end cooperation with the United States.
A spate of suspected U.S. missile strikes into the lawless border region and a Sept. 3 raid by U.S. commandos said to have killed 15 people have highlighted U.S. impatience and angered many Pakistanis.
In the latest such alleged breech, two U.S. helicopters crossed one mile late Sunday into Pakistan in the Alwara Mandi area in North Waziristan, two intelligence officials said on condition of anonymity because they were not authorized to speak to the media.
Citing informants in the field, they said Pakistani troops and tribesmen responded with small arms fire, but it was not clear whether the bullets were aimed at the choppers or were warning shots.
The helicopters did not return fire and re-entered Afghan airspace without landing, the officials said.
That account was denied by Pentagon officials.
"There was no such incursion, there was no such event," said Defense Department spokesman Col. Gary L. Keck.
Pakistan's army said it had no information on the reported incursion across the poorly demarcated border.
Pakistan's military chief and newly elected President Asif Ali Zardari have said the missile strikes and incursions were violations of the country's sovereignty and only fueled extremist violence.
Zardari, who is expected to meet U.S. President George W. Bush in New York on the sidelines of the U.N. General Assembly this week, reiterated that he welcomed U.S. intelligence help, but not its troops.
"Give us the intelligence and we will do the job," he said in an interview with NBC television. "It's better done by our forces than yours."
Some 270 people were wounded in the Saturday night attack on the heavily guarded Marriott hotel in the capital Islamabad, while the dead included the Czech ambassador and two U.S. Department of Defense employees.
Most of the victims were Pakistanis, a fact that could bolster government efforts to present the struggle against the militants as its own battle, not one foisted upon it by Washington as many here think.
Al-Arabiya television said the group that claimed responsibility for the attack demanded an end to Pakistani-American cooperation against the militants and a halt to U.S. military operations in Pakistani tribal regions.
The U.S. Embassy in Islamabad warned its employees to limit their movement to travel to and from the Embassy and to shopping for essential items only.
It also warned all Americans to stay away from crowds, keep a low profile, and avoid setting patterns by varying times and routes for all required travel.
In Washington, State Department spokesman Robert Wood said the attack showed the need for Pakistanis, Afghans, and the U.S. to redouble efforts against extremists in the region.
"This was a heinous act that was committed by terrorists who have no interest in anything other than maiming and killing innocent civilians. And we're going to step up our efforts and work with the Pakistanis to do what we can," he said.
Interior Ministry chief Rehman Malik said Zardari, the prime minister and other top government officials were due to dine at the Marriott on Saturday, but they decided to change venue at the last minute.
However, a spokesman for the hotel owner denied this.
"We didn't have any reservation of such a dinner that the government official is talking about," Jamil Khawar told The Associated Press.
Malik told reporters that "perhaps the terrorists knew" that the Marriott was the venue of the government dinner, saying the decision to switch venues "saved the entire leadership."
Malik had said Sunday the Marriott was likely targeted because the attack would get tremendous attention. But Prime Minister Yousuf Raza Gilani said the bomber attacked the hotel only after security kept him from reaching Parliament or the premier's residence, both less than a mile away.
Pakistani officials and analysts have said they expect the investigation into the hotel to lead to the border regions.
But Amir Mohammad, an aide to prominent Pakistani Taliban leader Baitullah Mehsud, said his chief was not involved and shared the nation's grief.
"We have our own targets and we execute our plans precisely with minimal loss of irrelevant or innocent people," Mehsud was quoted as saying by his spokesman.
Mehsud has also been blamed for the suicide attack that killed Zardari's wife, the pro-U.S. politician Benazir Bhutto.
Underscoring the deteriorating security, gunmen kidnapped Afghanistan's ambassador-designate Monday and killed his driver in the northwestern city of Peshawar.
Also in the northwest, a suicide bomber killed nine security officers at a checkpoint in the Swat valley, while police killed at least 10 militants in a gunbattle elsewhere, officials said.
British Airways said Monday it was temporarily suspending its flights to the country following the hotel attack.
The airline, which offered six flights to Pakistan each week, did not face a direct security threat, company spokesman Suhail Rehman said.
Meanwhile, Pakistan's army says a suicide car bomber killed nine troops in the country's volatile northwest. Army spokesman Maj. Murad Khan said the attacker detonated an explosives-laden vehicle at a checkpoint in Madian, a town in the Swat valley, on Monday. Three troops quickly succumbed to their injuries, and six more died despite the efforts of medics.