The rapid U.S. build up in the Afghan war will include more terrorist-hunting forces to chase down militants deemed too extreme to change sides, a top U.S. general revealed on Wednesday.

"There's no question you've got to kill or capture those bad guys that are not reconcilable," Gen. David Petraeus, the chief of U.S. Central Command, told the Senate Foreign Relations Committee. "And we are intending to do that."

In his first congressional testimony on President Barack Obama's announced plan to send another 30,000 troops to Afghanistan, Petraeus also cautioned that progress against probably will be slower than during the build up of U.S. forces in Iraq two years ago, and the war will be "harder before it gets easier."

Petraeus said that in addition to an effort to "reintegrate" Taliban and other insurgents into mainstream Afghan society, there will be a harder push to eliminate the most hardcore extremists.

"In fact, we actually will be increasing our counterterrorist component of the overall strategy," Petraeus said. He provided no details beyond saying that additional "national mission force elements" would be sent to Afghanistan next spring.

Petraeus appeared to be referring to classified units such as the Army's Delta Force that specialize in counterterrorism and that have been used extensively in both Iraq and Afghanistan. Gen. Stanley McChrystal, who now oversees the Afghan war and is scheduled to testify Thursday before a House committee, previously headed up those units inside Iraq and Afghanistan.

In an interview with PBS's Charlie Rose on Wednesday, McChrystal cited a formula for success in the counterterrorism effort he led in Iraq, saying the point is to eliminate the middle levels of terrorist networks like al-Qaida rather than focusing on killing only the most senior leaders.

"You cause the network to collapse on itself," McChrystal said. "And that's what I saw happen in Iraq, and that's one of the things we're working on in Afghanistan." He did not mention sending more counterterrorism forces to Afghanistan.

Much of Wednesday's Senate hearing focused on the link between instability in Afghanistan and the presence of Taliban, al-Qaida and other extremist groups in neighboring Pakistan.

Sen. Richard Lugar of Indiana, the ranking Republican on the committee, said he's confident allied forces will improve security in Afghanistan, but that the biggest question is whether that will help root out Taliban and al-Qaida havens across the border in Pakistan.