By David Morgan

WASHINGTON (Reuters) - U.S. Army Gen. David Petraeus said on Tuesday that security gains in Iraq are increasingly durable but warned that that methods which helped reduce violence there may not work in Afghanistan.

Petraeus, the former commander in Iraq who is credited by U.S. officials with saving the country from sectarian war, emphasized that progress in Iraq remains fragile and reversible despite an 80 percent drop in violence.

"But I have to say that the fragility is less," he said in a speech to the Association of the U.S. Army, a nonprofit group that supports the armed service.

"And with each passing day there is a little bit less of that fragility as progress takes on a slightly more enduring nature," he said.

It was an unusually upbeat assessment of the situation from the normally cautious Petraeus, who turned over command in Iraq to Gen. Ray Odierno last month.

Petraeus was the main architect of a 2007 build-up in U.S. forces known as the surge, which has been given much credit for reducing violence levels.

Violence also fell after Sunni tribesmen joined U.S. forces against al Qaeda and after radical Shi'ite cleric Moqtada al-Sadr declared a ceasefire for his militia.

Petraeus will begin overseeing U.S. operations in Afghanistan on October 31 when he takes over U.S. Central Command, which is responsible for U.S. military interests across the Middle East and into south and central Asia.

EVER-CHANGING INSURGENCIES

But the general said methods used to quell violence in Iraq may not be transferable to the war in Afghanistan, where attacks by the Taliban and other militant groups is surging.

"What worked in Iraq may not work in Afghanistan" because of the unique and ever-changing nature of insurgencies. "What works in Baghdad today will not work in Baghdad tomorrow. What works in Baghdad may not work in Falluja," he said.

The drop in violence in Iraq has allowed the Bush administration to announce the withdrawal of 8,000 troops from the country by early next year and to begin moving other forces to Afghanistan.

But with the danger of renewed violence heightened by the approach of provincial elections in January, the United States is unlikely to begin sending thousands more soldiers sought by NATO commander in Afghanistan, David McKiernan, until next spring or summer.

There are currently 155,000 U.S. forces in Iraq and 33,000 in Afghanistan, 13,000 of which are under NATO command.

Petraeus said the flow of foreign fighters from Syria to Sunni insurgent groups including al Qaeda in Iraq has declined to 10-20 people a month from a height of 160.

U.S. troops have also recently seen a sharp decline in the number of weapons caches they are finding after big increases since 2006. "We think we are literally running out of safe havens and strongholds and starting to run out of these areas where there were these very significant caches," Petraeus said.

(Editing by Cynthia Osterman)

Petraeus sees increasingly durable gains in Iraq -  Source