But the officials said on Friday they did not know whether the fighters were protecting a senior al-Qaida leader despite the Pakistani belief that a "high value target" - possibly al-Qaida number two Ayman al-Zawahiri - was the object.
"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, chairman of the Joint Chiefs of Staff, said in an interview with Fox television.
Myers said Pakistan had not asked for US military help in the operations, which began earlier this week and continued on Friday in South Waziristan, a tribal area along the Pakistani-Afghan border US officials said had long been a haven for al-Qaida 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.
Still not clear if al-Qaida No 2,
Ayman al-Zawahri is in the area
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.
A defence official said Myers was not necessarily referring to more troops being sent to the border, but rather to closer surveillance of the area.
"It's not as if we have our hands tied together preventing people from getting through," he said. "It's safe to say we are watching and monitoring the border over there."
Meanwhile, National Security Adviser Condoleezza Rice also sought to downplay expectations, telling CBS television even if al-Zawahiri were captured it would not spell the end of al-Qaida.
Condoleezza Rice says Zawahri's
capture will not spell al-Qaida's end
"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," she said. "But I don't think we should jump to the conclusion that it would ... completely disable al-Qaida."
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 are working to mount "hammer-and-anvil" operations in which US and Afghan forces would block al-Qaida fighters moving out of the way of Pakistani forces.
High tech help
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-Qaida fighters had 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 Pakistani 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-Qaida and some of their local supporters," he said.
"I'm sure Barno has been in good contact with the Pakistanis," said Pentagon spokesman Larry DiRita, adding, "We don't have anyone in there. No DoD military are involved over there."
Officials would not comment on reports that CIA-led teams are operating with the Pakistanis.