WASHINGTON — Pentagon leaders yesterday signaled a surge in American forces in Afghanistan “sooner rather than later” — a shift that could come later this year as they prepare to cut troop levels in Iraq.

Faced with an increasingly sophisticated insurgency, particularly along Afghanistan’s border with Pakistan, defense officials said sending more troops would have a significant impact on the violence.

“I think that we are clearly working very hard to see if there are opportunities to send additional forces sooner rather than later,” Defense Secretary Gates told Pentagon reporters. But, he added that no final decisions or recommendations have been made.

His comments suggested an acceleration in what had been plans to shift forces there early next year. And they came as the political discourse on Afghanistan as a key military priority escalated on both Capitol Hill and the presidential campaign trail.

The chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, who recently returned from meetings with commanders in Afghanistan, said they clearly want more troops now.

“It’s a tougher fight, it’s a more complex fight, and they need more troops to have the long-term impact that we all want to have there,” Admiral Mullen, who also met last week with Pakistani leaders, said.

The Pentagon has been wrestling with how to provide what they say is a much needed military buildup in Afghanistan, while they still have 150,000 troops in Iraq. Mr. Gates and Admiral Mullen have repeatedly said they would have to reduce troop levels in Iraq before they could dedicate more forces to Afghanistan.

Admiral Mullen, who was in Iraq last week, told reporters that he is likely to recommend further troop reductions there this fall. He said he found that conditions in Iraq had improved more than he expected.

“I won’t go so far as to say that progress in Iraq from a military perspective has reached a tipping point or is irreversible — it has not, and it is not,” Admiral Mullen told a Pentagon press conference.

“But security is unquestionably and remarkably better. Indeed, if these trends continue, I expect to be able early this fall to recommend to the secretary and the president further troop reductions,” he said.

The military buildup in Iraq that began more than 18 months ago has ended, now that the last of the five additional combat brigades sent in by President Bush last year has left the country.

Its departure marks the end of what the Pentagon called the “surge.” And it starts the 45-day evaluation period that the top American commander in Iraq, General David Petraeus, told Congress he would need to assess the security situation and determine how many more troops he could send home.

Neither Mr. Gates nor Admiral Mullen would detail how they intend to juggle the military requirements in Iraq and Afghanistan, they spoke more aggressively about meeting Afghan needs more quickly.