WASHINGTON - There is a growing consensus among defense leaders to send a substantial contingent of Marines to Afghanistan, probably beginning next spring, while dramatically reducing their presence in western Iraq, the top Marine general said yesterday.

Gen. James Conway, the Marine Corps commandant, said in an interview that Marine units tentatively scheduled to go to Iraq next spring were already incorporating some training for Afghanistan into their preparations.

He said he had talked with Defense Secretary Robert M. Gates and believed the Pentagon chief "would not object to the idea of a fairly strategic shift of focus of Marines from Iraq to Afghanistan."

"I don't want to put words in his mouth," said Conway, who has made no secret of his belief that Marines could be put to better use fighting in Afghanistan than their current peacekeeping, nation-building mission in Iraq.

Gates understands, he said, "my public stance on the fact that we can be better used elsewhere. And he certainly hasn't told me to pipe down. So I like to think he understands the logic of it."

At the same time, Conway said that when the 22,000 Marines in Iraq's Anbar province leave, he believes they all should go, and not leave training teams behind.

More than a year ago, when early discussions of sending more Marines to Afghanistan became public, Gates signaled opposition to the idea, preferring to maintain the concentration on Iraq.

At that time, Conway said Gates and others believed the timing was not right to shift the Marines out of Anbar.

Yesterday, Conway took a decidedly different tone.

"I just see that people have, over time, understood we don't want to take over Afghanistan, such as was rumored when we first started talking about a shift of forces," Conway said.

Instead, he said, officials now realize that the Marines are an expeditionary fighting force better suited to the Afghanistan battle.

In an illustration of the growing challenge for the United States in Afghanistan, an international think tank estimated in a report released yesterday that the Taliban had a "permanent presence" in nearly three-quarters of the country.

The report by the International Council on Security and Development described the Taliban as "the de facto governing power" in some towns and villages in southern Afghanistan, and it said the extremist group had managed to advance into Afghanistan's western and northwestern provinces, as well as some areas north of Kabul, the capital.

Gen. David McKiernan, commander of U.S. and NATO troops in Afghanistan, has said he needs up to 20,000 additional troops, including four combat brigades and thousands of support troops.

Other military leaders have cautioned, however, that they first must build the infrastructure to accommodate the troops - including housing and helicopter pads.

Conway countered that the Marines could move in far more quickly because they do not need to wait for such logistical improvements.

"We're prepared to live austere for a time in order to take the fight to the enemy and build our infrastructure around us on deck," Conway said. "We have done that before; we can do it again."