Washington ? Faced with waning public support for the military escalation in Afghanistan, Defense Secretary Robert Gates said Thursday that the war is worth fighting and signaled for the first time he may be willing to send more troops after months of publicly resisting a significant increase.

Gates urged patience amid polls showing rising disenchantment among the public with the war effort, saying the American military presence in Afghanistan was necessary to derail terrorists.

At a Pentagon news conference, Gates said efforts by President Barack Obama — including ordering an additional 21,000 U.S. troops to Afghanistan this spring — are “only now beginning” and should be given a chance to succeed.

“I don’t believe that the war is slipping through the administration’s fingers,” Gates said. Later, he added: “I absolutely do not think it is time to get out of Afghanistan.”

At the same time, there is a “limited time for us to show that … this approach is working,” Gates said.

Sitting beside Gates, Joint Chiefs Chairman Adm. Mike Mullen described “a sense of urgency” in securing Afghanistan to make sure extremists can no longer hatch terrorist plots against the United States and its allies from within its borders.

“Time is not on our side,” Mullen said, adding that the military mission in Afghanistan until recently has been underfunded and undermanned. “Part of why it has gotten more serious and has deteriorated has been directly tied to that.”

Both Gates and Mullen declined to talk about any of the recommendations contained in a new review of Afghanistan strategy sent this week to them and the president. Gates said only he could consider a major increase in combat troops under certain conditions.

Gates said he would be comfortable with a larger U.S. military presence in Afghanistan as long as the increase reassured the country’s citizens that the Americans were there for the benefit of Afghans.

“If they interact with the Afghans in a way that gives confidence to the Afghans that we’re their partners and their allies, then the risks that I have been concerned about the footprint becoming too big and the Afghans seeing us in some role other than partners I think is mitigated,” Gates said.

A separate recommendation on troop increases is expected in the coming weeks from the top commander in Afghanistan, Gen. Stanley McChrystal, who wrote the new review, but how many troops McChrystal wants is unclear.

There could be as many as 20,000, but in recent days military officials have predicted it will be far less, closer to or fewer than 10,000.

Mullen said the question of a new jump in troop deployments is just one element of a larger plan that the Pentagon will soon ask Congress to authorize. “It’s a piece — critical, but it’s not total,” Mullen said.

Despite recent calls from leftist activists and also from conservative columnist George Will to wind down U.S. military involvement in Afghanistan, Gates forcefully argued for continued American efforts there.

Fifty-one U.S. troops died in Afghanistan in August, making it the bloodiest month for American forces there since the U.S.-led invasion in late 2001.

Gates cited the continuing threat from al-Qaida and its Taliban allies as the top reason why the U.S. should stay in Afghanistan. Leaving would allow terrorists to re-establish staging bases in a nation where the political leadership is unable to curb insurgent threats, Gates said in a blunt reference to the Sept. 11, 2001, attacks.