WASHINGTON (Reuters) - The Pentagon decried overly negative assessments of the Afghan war on Wednesday, telling Congress the conflict is a “roller coaster” of ups and downs but insisting progress is being made.

Commander of the U.S. Central Command, Army General David Petraeus (R), testifies on the situation in Afghanistan during a hearing held by the Senate Armed Services Committee on Capitol Hill in Washington June 16, 2010, as Defense Undersecretary for Policy Michele Flournoy looks on. The United States has presented evidence to Pakistan that the Haqqani militant faction aligned with the Taliban and based in Pakistan orchestrated brazen attacks last month in Afghanistan, Petraeus said on Wednesday. REUTERS/Kevin Lamarque

President Barack Obama and U.S. military planners are on the defensive due to growing anxiety over a six-month-old strategy that lawmakers fear is failing to turn the tide in the costly, unpopular conflict.

The strategy hinges on pouring U.S. forces into southern Afghanistan, the heartland of the Taliban insurgency, before starting a gradual withdrawal in July 2011, conditions permitting.

Defense Secretary Robert Gates acknowledged it would be a “long and a hard fight” and that U.S. casualties would rise over the summer. But he assured skeptical lawmakers the new strategy had put the Afghan war on the right path.

“I think frankly that the narrative ... has been too negative. I think that we are regaining the initiative. I think that we are making headway,” Gates said.

Gates, a holdover from the George W. Bush administration, called for patience amid tough questions about the effectiveness of the war strategy from members of Congress, including Obama’s fellow Democrats. He said progress could not be measured week-to-week.

“This is not something where we do ourselves any favors by tearing ourselves up by the roots every week to see if we’re growing,” Gates told the Senate Appropriations Committee.

Senator John McCain, a top Republican who lost to Obama in the 2008 presidential election, said on Tuesday he was deeply concerned about the trends in the Afghan conflict.

McCain warned the U.S. campaign could be heading toward a “crisis” and expressed skepticism over the July 2011 timeline, given the violence and lawlessness on the ground.

The second-guessing has touched a nerve in the Pentagon, where some worry negativity is undercutting public sentiment for Obama’s strategy before it has a chance to work. About a third of the 30,000 reinforcements Obama deployed in December have yet to arrive in Afghanistan.

“I have a certain sense of deja vu,” Gates said. “I was sitting here getting the same kind of questions about Iraq in June of 2007, when we had just barely gotten the surge forces into Iraq at that point.”

ROLLER COASTER

General David Petraeus, who oversees the Afghan war as head of U.S. Central Command, compared the campaign to a roller coaster ride. But overall, the situation was improving, he told the Senate Armed Services Committee.

“It is truly an ‘up and down’ (experience), when you’re living it, when you’re doing it,” Petraeus said. “But the trajectory in my view has generally been upward, despite the tough losses, despite the setbacks.”

Asked later by the House of Representatives Armed Services Committee, Petraeus said: “Defining winning as making progress, then I think we are winning in Afghanistan. It is a roller coaster ride, however.”

A lone protester interrupted the hearing, shouting “This is mass murder” as she was being escorted out by police.

To bolster his case to the U.S. public, Obama needs to show progress in Afghanistan by December when a White House review of the war strategy is scheduled.

Perceptions of a struggling U.S. campaign have been fueled by stronger-than-expected Taliban resistance in the southern district of Marjah -- meant to be a showcase of U.S. strategy -- and a slower start to a long-awaited offensive in the Taliban’s birthplace of Kandahar.

A massive military operation in Kandahar is the linchpin of Obama’s campaign to turn the tide in the war this year and will weigh heavily in a White House review of strategy.

Michele Flournoy, undersecretary of defense for policy, said media reporting about “the so-called delay in the Kandahar campaign has been overplayed.”

“I think we owe General (Stanley) McChrystal a great deal of operational flexibility,” she said, referring to the top NATO and U.S. commander in Afghanistan.

Some of the deepest concerns center on Afghan President Hamid Karzai’s government. Karzai has made repeated anti-Western statements and raised eyebrows last weekend by firing his interior minister and intelligence chief, two cabinet members broadly respected by Washington.

Gates said he had confidence in Karzai, his government and Afghan security forces.