By Missy Ryan and Susan Cornwell
WASHINGTON (Reuters) - The commander of U.S. and NATO forces in Afghanistan delivered an optimistic assessment of the unpopular war on Tuesday, telling lawmakers his troops will continue to make headway against Taliban insurgents even with a drawdown of American troops set to begin in July.
General David Petraeus, in his first congressional testimony since becoming Afghanistan commander last year, reiterated the Pentagon's message that President Barack Obama's decision to deploy an extra 30,000 troops succeeded in winning key territory away from the Taliban.
"There have been setbacks as well as successes. Indeed, the experience has been akin to that of a roller coaster ride," Petraeus told senators. "Much difficult work lies ahead."
"Although the insurgents are already striving to regain lost momentum and lost safe havens as we enter the spring fighting season, we believe that we will be able to build on the momentum achieved in 2010 -- though that clearly will entail additional tough fighting," Petraeus added.
American troops have been fighting in Afghanistan since 2001, when forces helped oust the Taliban rulers who had harbored the al Qaeda militants responsible for the September 11 attacks on the United States.
Yet Petraeus faces a challenge in arguing that the U.S. military can safely reduce its numbers in Afghanistan even as it prepares for a bloody spring and the Pentagon urges NATO nations against pulling out too quickly.
Currently there are almost 100,000 U.S. troops in Afghanistan and Obama has promised to begin bringing some of them home starting in July.
While Petraeus presented senators a litany of accomplishments, he acknowledged progress remained "fragile and reversible" in Afghanistan, where the Taliban has been weakened in parts of its southern heartland but where they also have moved into previously quiet areas.
Public support has waned for the war, which has killed almost 1,500 U.S. soldiers since 2001 and now costs over $100 billion a year.
Nearly two-thirds of Americans now think the war is not worth fighting, a Washington Post/ABC News poll showed on Wednesday.
Obama, facing pressure to create jobs and rein in the U.S. debt, will be mindful of such numbers as he looks toward the 2012 election.
Lawmakers voiced concerns about the future in Afghanistan even if military progress can be sustained.
Governance remains weak across Afghanistan, where a rift has grown between Washington and Afghan President Hamid Karzai over civilian deaths and over rampant corruption.
Petraeus said U.S. and NATO troops would be thinned out and reassigned within Afghanistan and local forces put in the lead for security in certain parts of the country.
Karzai is expected to announce next week which areas can be shifted initially to Afghan control, despite persistent weaknesses in local forces, including high rates of illiteracy, drug abuse, attrition and insurgent infiltration.
"As we embark on the process of transition, we should keep in mind the imperative of ensuring that the transition actions we take will be irreversible," Petraeus said. "We'll get one shot at transition, and we need to get it right."
Senator John McCain, a key Republican voice on military affairs, warned against bowing to political pressure for an early withdrawal and said U.S. commanders should be "exceedingly cautious" about withdrawing troops this July.
"We should not rush to failure and we should cultivate strategic patience," McCain said.
Last week, Defense Secretary Robert Gates told other countries with troops in Afghanistan they could endanger security progress by withdrawing their troops too quickly.
Petraeus said he could "understand" public frustration over the war but said that al Qaeda -- who he said had less than 100 fighters in Afghanistan currently -- would seek to strengthen its presence in Afghanistan if foreign troops were to cede more territory to the Taliban.
(Editing by Eric Beech)