Pakistani troops and tribesmen opened fire on two U.S. helicopters that crossed into the country from neighboring Afghanistan, intelligence officials said Monday. The U.S. denied the report.

The helicopters did not return fire and re-entered Afghan airspace without landing, the officials said, speaking on condition of anonymity because they were not authorized to speak to media.

"There was no such incursion, there was no such event," said Col. Gary L. Keck, Defense Department spokesman.

The reported incursion late Sunday will likely add to tensions between Islamabad and Washington.

A spate of suspected U.S. missile strikes into Pakistan's border region and a raid by U.S. commandos said to have killed 15 people have angered and embarrassed Pakistani leaders while signaling Washington's impatience with Pakistani efforts to clear out militant havens.

During a recent speech to Parliament, newly elected President Asif Ali Zardari, who is considered U.S.-friendly, warned that no country would be allowed to violate Pakistan's sovereignty in the name of the war on terror.

Zardari is on his way to New York to attend the U.N. General Assembly, and he is expected to meet President Bush.

The two intelligence officials said informants in the field told them the incursion was about a mile inside the disputed and poorly demarcated border in the Alwara Mandi area in North Waziristan, a tribal region the U.S. considers a sanctuary for Taliban and al-Qaida fighters.

A week ago, U.S. helicopters reportedly landed near Angoor Ada, a border village in nearby South Waziristan, but returned toward Afghanistan after troops fired warning shots.

A Pakistani military spokesman said last week that troops had orders to open fire in case of another cross-border raid by foreign troops.

The U.S. cross-border attacks have angered Pakistanis and analysts said the weekend suicide bombing of the Marriott hotel in Islamabad may have been a warning from al-Qaida and the Taliban to the Pakistani government to end cooperation with U.S. in fighting militants. The attack prompted foreign diplomatic missions and aid groups in Pakistan to review their security status.

The U.S. Embassy in Islamabad warned its employees Monday to limit their movement to travel to and from the Embassy and to shopping for essential items only. American consulates in Lahore and Peshawar reminded their personnel to avoid large hotels in those cities. The Embassy 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.