The U.S. troop drawdown in Iraq is a "truly remarkable achievement" and demonstrates President Barack Obama's commitment to responsibly ending the drawn-out conflict, the White House said Tuesday.

The assessment from White House counterterrorism chief John Brennan came after the Pentagon reported that U.S. troop strength in Iraq has dropped below 50,000 for the first time since the 2003 U.S.-led invasion.

Briefing reporters as the president vacationed on Martha's Vineyard, Mass., Brennan noted that the milestone had been reached a week ahead of schedule and represented a drop of 94,000 troops on Obama's watch.

Obama plans a major speech next week to mark the formal end of the combat mission in Iraq, though a substantial U.S. contingent will remain in a training and backup role.

Deputy Press Secretary Bill Burton said plans for the speech are still being drawn up. But he said Obama will address "the importance of the milepost," praise the heroism of U.S. troops who've fought and died there and outline the administration's continuing goals.

Brennan said the removal of tens of thousands of U.S. troops without destabilizing Iraq "represents a truly remarkable achievement for our military and for the country."

Asked if America will still be at war in Iraq once the combat mission ends, Brennan replied "right now we still have two theaters of military operations: Iraq and Afghanistan."

Brennan acknowledged that the Iraqis still face sizable challenges, from forming a stable government to preventing terrorist bombings.

"There's still more progress that needs to be made inside of Iraq to ensure that security is going to prevail throughout the country an is going to be enduring," he said.

Brennan said terrorist groups like al-Qaida in Iraq "will try to argue that they have been successful" in pushing U.S. troops out. "But they are wrong. We are reducing our footprint in Iraq under our terms."

Brennan praised Iraqi security forces, which he said were showing increasing competence. Those forces "have increasingly moved into the lead," he said.

"Although these terrorists continue to kill innocent Iraqis," Brennan said, "they have failed to ignite sectarian violence and violence continues to be at a reduced level."

Back in Washington, Vice President Joe Biden echoed Brennan's assessment of the troop reduction.

"This, quite frankly, is a remarkable milestone in a war that began more than seven years ago ... the second-longest war in American history," said Biden, who has become an administration point-man on Iraq. "We owe an incredible debt of gratitude to the tens of thousands of American troops who have sacrificed to get us to this place."