(CNN) -- As coalition and Afghan forces entered the second week of a major offensive against the Taliban in southern Afghanistan, the head of U.S. Central Command warned that the potential loss of lives among U.S. forces in the operation "will be tough."

Gen. David Petraeus said the losses could be comparable to those seen after the 2007 surge of U.S. troops into Iraq.

"They'll be tough. They were tough in Iraq," Petraeus said on NBC's "Meet the Press."

From the time the U.S. surge in Iraq began in January 2007 until its official end in July 2008, 1,125 U.S. troops died -- more than a quarter of the total 4,379 Americans who have died in the Iraq war.

"The reality is that it's hard, but we are [in Afghanistan] for a very, very important reason, we can't forget that," he added. "We're in Afghanistan to ensure that it cannot once again be a sanctuary for the kinds of attacks that were carried out on 9/11."

Despite stiff Taliban resistance to Operation Moshtarak in Helmand province, Petraeus said that the militants are "a bit disjointed at this point in time."

"When we go on the offensive, when we take away sanctuaries and safe havens from the Taliban and other extremist elements that we and our Afghan and coalition partners are fighting in that country, they're going to fight back," he added. "And we're seeing that in Marjah, we will see that in other areas, but we are going after them across the spectrum."

Petraeus noted that the offensive is just the "initial salvo" of what will be a 12- to 18-month military campaign, but results are being seen already.

"We have more of our special operations forces going in on the ground, and you've seen the results, you've heard some of the initial results of that with more ... Taliban shadow governors being captured, more of the high-value targets being taken down."

See more Afghanistan coverage at Afghanistan Crossroads blog

The Central Command chief also addressed his group's assessment of al Qaeda, following a dust-up a week ago between former Vice President Dick Cheney and current Vice President Joe Biden.

Biden had argued that another massive terror attack against the United States, like the one on September 11, 2001, was "unlikely."

But Cheney called that analysis "dead wrong," and said the biggest threat facing the United States now is a potentially huge terror attack with nuclear weapons or biological agents.

Petraeus said the assessment of the U.S. Central Command is that al Qaeda has been "diminished" in the past year.

"But ... al Qaeda is a flexible, adaptable -- it may be barbaric, it may believe in extremist ideology as it does -- but this is a thinking, adaptive enemy and we must maintain pressure on it everywhere."