* U.S. troops exchange fire with Pakistani forces

* Zardari says won't let 'friends' violate sovereignty

* Rice says U.S. to work closely with Zardari government

By David Morgan

WASHINGTON, Sept 25 (Reuters) - U.S. and Pakistani ground forces exchanged fire across the Afghanistan-Pakistan border on Thursday, the latest in a string of incidents that has ratcheted up diplomatic tension between the two allies.

No casualties or injuries were reported after Pakistani forces shot at two U.S. helicopters from a Pakistani border post. U.S. and Pakistani officials clashed over whether the American helicopters had entered Pakistan.

The incident follows a U.S. campaign of attacks on militant targets inside Pakistan, including a Sept. 3 U.S. commando raid on a village compound in South Waziristan. Islamabad has protested those strikes and warned it would defend itself.

"Just as we will not let Pakistan's territory be used by terrorists for attacks against our people and our neighbors, we cannot allow our territory and our sovereignty to be violated by our friends," Pakistani President Asif Ali Zardari said in New York on Thursday.

But in Washington, Pentagon spokesman Bryan Whitman insisted the helicopters had not entered Pakistan. He described the incident as "troubling" and called on Islamabad for an explanation.

"The flight path of the helicopters at no point took them over Pakistan," he said. "The Pakistanis have to provide us with a better understanding of why this took place."

According to Pakistan's military, its soldiers fired warning shots at two U.S. helicopters after they intruded into Pakistani airspace. The U.S. military said the helicopters were protecting a patrol about one mile (1.6 km) inside Afghanistan when Pakistani forces opened fire.

UNCERTAIN BORDER

"The (helicopters) did not return fire but the ground forces fired suppressive fire at that outpost. The Pakistani forces then returned that fire. The whole exchange lasted about five minutes," said an official with U.S. Central Command, which oversees American military operations in Afghanistan.

The U.S. forces were operating under NATO command.

Thursday's confrontation followed a dispute earlier this week over reports of a downed U.S. drone in Pakistan. Pakistani officials said a small unmanned American aircraft crashed in Pakistan, but U.S. officials denied it, saying a drone went down in Afghanistan and was recovered.

The rugged border region between Afghanistan and Pakistan is seen by Washington as critical to its fight against al Qaeda and the Taliban. The Bush administration considers Pakistan an ally in counterterrorism but U.S. officials say Islamabad has not done enough against militants there.

The uncertain border also complicates efforts, making it difficult for forces to determine when they are in Afghanistan or Pakistan, both U.S. and Pakistani officials concede.

U.S. Secretary of State Condoleezza Rice, after meeting Zardari on Thursday, said she believed he was strongly committed to fighting militants.

"We talked about how we might assist Pakistan in doing what it needs to do, but I think there is a very strong commitment. And after all, it is the same enemy," she said in New York on the sidelines of the U.N. General Assembly. (Additional reporting by Zeeshan Haider in Islamabad, Sue Pleming in New York and Jonathon Burch in Kabul; Editing by Kristin Roberts and Eric Walsh)

AlertNet news is provided by