ISLAMABAD, Pakistan - Assailants fired at a U.S. military helicopter Tuesday as it ferried supplies to earthquake victims in Pakistan's portion of divided Kashmir, the U.S. military said, but it vowed to continue aid flights. The attack with an apparent rocket-propelled grenade came as the CH-47 Chinook flew over Chakothi, a quake-ravaged town near the frontier separating the Pakistani and Indian portions of the Himalayan region, said Capt. Rob Newell, a spokesman for the U.S. military relief effort. "The aircraft was not hit and returned safely with its crew" to an air base near the capital, Islamabad, he told The Associated Press. The Pakistani army spokesman, Maj. Gen. Shaukat Sultan, expressed skepticism an attack took place, saying engineers were using explosives to clear a road near where U.S. helicopters were flying. "The blast was huge enough to kick up dust which the pilot probably misunderstood as rocket fire," Sultan said, adding that Pakistani soldiers searched the area after the reported attack and witnesses on the ground did not see a rocket attack. A U.S. military officer, speaking on condition of anonymity because of the sensitivity of the subject to Pakistan-American relations, said the military stood by its account. He said the crewman who reported the attack had just served in Afghanistan for months and can recognize a fired rocket-propelled grenade. Kashmir — one of two Pakistani areas hit hardest by the Oct. 8 quake — is a focus for Islamic separatists fighting in India's part of the region. While Pakistan denies militants use its territory as a base, their presence is barely hidden. While most Pakistanis have expressed gratitude for U.S. help since the quake, some question their intentions. There are rumors that U.S. troops are conducting reconnaissance operations, taking pictures of Pakistan's nuclear facilities or searching for al-Qaida militants. Newell said the attack would not weaken America's determination to help Pakistan recover from the quake, which is believed to have killed about 80,000 people and left more than 3 million homeless. "We are going to continue to fly the helicopter missions in support of the relief effort and in support of Pakistan," he said. His comments followed a pledge by Rear Adm. Mike LeFever, commander of the U.S. disaster assistance center near Islamabad, that U.S. helicopters will keep flying aid missions through the winter. Twenty-nine are now in action, mostly heavy-lifting C-47s. "This is long-term support. We are going to be standing by our friends, and we expect the other international communities to be able to do that," LeFever told reporters in Muzaffarabad, the region's main city. Also Tuesday, the U.N. reiterated a warning that all 23 of its helicopters could be grounded within a week unless adequate aid money comes through quickly to pay for the flights. "The warning persists. If we do not receive any funds, then we have to ultimately ground the helicopters," said Amjad Jamal of the World Food Program, one of the U.N. agencies taking part in the relief effort. In Washington, Gen. Peter Pace, chairman of the Joint Chiefs of Staff, expressed pride in the U.S. aid effort. "Right now we have over 800 U.S. armed forces on the ground, side by side with their Pakistani counterparts; over 24 medium- and heavy-lift helicopters, with nine more on the way," he said. "Fixed-wing airplanes are dropping relief supplies. Almost 4,000 tons of relief supplies have been delivered." LeFever stressed the importance of continuing helicopter operations "throughout the winter months that are coming." Helicopters have proved crucial for ferrying supplies and recovering injured people in towns and villages cut off by landslides — and are likely to remain so for an extended time. Pakistan's army said Tuesday that it could not predict when a key road in Kashmir would be reopened because of landslides, complicating plans to get supplies to hundreds of thousands of people without shelter, some already facing freezing temperatures in upland villages. Relief workers are rushing to deliver tents, food and medicine before villages are cut off by snow and helicopter-grounding fog. Nighttime temperatures in Muzaffarabad were forecast to dip to around 44 degrees, while snow was expected in villages above 11,000 feet, with temperatures as low as 10 in the highest settlements. Sultan, the Pakistani army spokesman, meanwhile, reported that a 26-year-old man was pulled alive from the rubble of his collapsed home in northern Pakistan on Monday, more than three weeks after the quake. But a doctor treating the man for severe dehydration and minor cuts said he doubted the claim. "You can live without water for a maximum of 10 days. It doesn't seem to be possible that he lived for so many days and we have no evidence of how he would have gotten food or water under the rubble," said Dr. Iftikhar Ahmed at the Ayub Medical Complex Hospital in Abottabad.