The US military reinforced the Afghan border on Friday as Pakistani troops fought pitched battles with suspected Al-Qaeda fighters in an ungoverned tribal area on the other side of the rugged frontier, senior US officials said.

But the officials here were unanimous in saying they did not know whether the fighters were protecting a senior Al-Qaeda leader despite the Pakistani's belief that a "high value target" -- possibly Ayman al-Zawahiri, Al-Qaeda's number two -- was among them.

"I think we are still waiting for the situation to continue to unfold, still trying to learn exactly what's happening on the ground," General Richard Myers, the chairman of the Joint Chiefs of Staff, said in an interview with Fox television.

Myers said Pakistan has not asked for US military help in the operations, which began earlier this week and continued Friday in South Waziristan, a tribal area along the Pakistani border that US officials said has long been a haven for Al-Qaeda fighters.

What we are doing, since we're very active on the Afghanistan side of the border, is that we're reinforcing that side of the border, and that's helpful," Myers told ABC in another interview.

"But this is a Pakistani operation and they have not asked for help as of yet," he said.

The general described the area as "very, very difficult terrain" with an open border, crossed at will by tribal groups who are "not particularly friendly."

"It's a difficult, difficult tactical situation," he said.

Meanwhile, National Security Adviser Condoleezza Rice also sought downplay expectations, telling CBS television that even if al-Zawahiri were captured it would not spell the end of Al-Qaeda.

"Obviously, when you can kill or capture one of the major leaders -- and Zawahiri is clearly one of their major leaders -- it would be a boost to the war on terrorism. But I don't think we should jump to the conclusion that it would by any means completely disable Al-Qaeda," she said.

US forces this week launched an offensive on the Afghan side of the border after weeks of intense preparations in which senior US, Afghan and Pakistani officials shared intelligence and sorted out procedures for coordinating their respective moves.

Lieutenant General David Barno, the US commander in Afghanistan, has said the two sides were working to mount "hammer-and-anvil" operations in which US and Afghan forces would block Al-Qaeda fighters moving out of the way of Pakistani forces.

A wide assortment of US aerial surveillance assets are trained on the border area, including unmanned Predator reconnaissance planes, RC-135 Rivet Joint aircraft capable of pinpointing electronic signals, and E8-C Joint Stars radar planes that follow the movement of vehicles on the ground.

It was not immediately clear whether any Al-Qaeda fighters have moved since Pakistani forces first clashed with them on Tuesday in a fierce battle that left at least 15 Pakistani soldiers dead.

"It continues to be a fluid situation in South Waziristan where fighting is continuing between Pakistan forces and these fighters," a US counter-terrorism official said.

"There is a pitched battle underway and the Pakistanis are continuing military operations against what are believed to be Al-Qaeda and some of their local supporters," he said.

"I'm sure Barno has been in good contact with the Pakistanis," said Larry DiRita, the Pentagon's chief spokesman, referring to Lieutenant General David Barno, the US commander in Afghanistan. But "we don't have anyone in there," he said.

"No DoD military are involved over there," he said.