U.S. forces may have again made a foray into Pakistan from neighboring Afghanistan, despite warnings from Islamabad.

Video: Pakistani Leaders Targeted?

Two intelligence officials said Pakistani troops and tribesmen opened fire on two U.S. helicopters after they crossed from Afghanistan into the northwest tribal region, where Taliban and al-Qaida militants are operating.

Intelligence officials said the helicopters didn't shoot back and flew back into Afghan airspace. The reported incursion was in North Waziristan, the tribal region that the U.S. considers to be a Taliban and al-Qaida sanctuary.

Last week, a Pakistani military spokesman said his country's troops had orders to open fire in case of another cross-border raid by foreign troops. And Pakistan's new president recently warned that no country would be allowed to violate Pakistan's sovereignty in the name of the war on terror -- including the United States.

In the meantime, Pakistan government officials said Monday that the country's top leaders were to dine at the Marriott Hotel devastated by a truck bombing over the weekend, but changed the venue at the last minute.

A hotel official, however, denied there were any plans for a government dinner.

The blast in the capital Islamabad killed at least 53 people and underscored the extremist challenge facing nuclear-armed Pakistan.

A U.S. Embassy spokesman in Pakistan said two Defense Department employees were among those killed. He said a third American, a contractor for the State Department, is unaccounted for.

No group has claimed responsibility for the attack, but officials and experts said the scale of the blast and its high-profile target are the hallmarks of media-savvy al-Qaida.

But a former government security chief for Pakistan's tribal areas, said that while it had "all the signatures" of an al-Qaida strike, homegrown Taliban militants probably have learned how to carry out such an attack.

He suggested al-Qaida is providing "money, motivation, direction" and leadership and, "using the Taliban as gun fodder."

The Pakistani leader will be meeting with President George W. Bush at this week's U.N. gathering in New York.