WASHINGTON: President Barack Obama's administration said that a July 2011 target date to begin withdrawing US troops fromAfghanistan was not set in stone, while stepping up warnings over corruption.

The day after Obama unveiled his new plan for turning around the war in Afghanistan with a surge of 30,000 more US soldiers, top officials stressed that any timetable for an eventual pullout was still flexible.

Secretary of State Hillary Clinton, Defense Secretary Robert Gates, and the top uniformed US military officer, Admiral Michael Mullen, sought to sell the new approach under fire from Obama's hawkish Republican foes.

During hours of questioning by two key committees, they made clear that his target date of starting a US troop withdrawal in 19 months' time -- a step some anti-escalation lawmakers, especially Democrats, had cheered -- could slip.Related article: Training key to victory in Afghanistan

"I do not believe we have locked ourselves into leaving," said Clinton, who added the goal was "to signal very clearly to all audiences that the United States is not interested in occupying Afghanistan."

Gates said the extra troops Obama had ordered to Afghanistan would be in place in July 2010, that a December 2010 review of the war effort would shape the pace of the withdrawal, and that the target date could change.

"I think the president, as commander in chief, always has the option to adjust his decisions," he told Republican Senator John McCain, Obama's defeated White House rival in 2008.

"Then it makes no sense for him to have announced the date," said McCain, the top Republican on the Senate Armed Services Committee.

Gates said he signed the formal deployment orders on the president's flight back to Washington from the West Point military academy.

"I signed the deployment orders last night on Air Force One coming back from West Point," Gates said in an interview with PBS television.

The defense secretary acknowledged that there would be more casualties as more US forces take the fight to insurgents, with the eight-year war already hitting record-high levels of violence.

"We lost 44 heroes in October; we lost 15 in November. So as we go into the winter, there may be fewer casualties, but certainly, there will be more casualties," Gates told "The Newshour with Jim Lehrer."

"And as spring comes and we are more engaged around the country, the casualties are almost certain to rise again."

Meanwhile, the White House warned Afghan President Hamid Karzai to fight corruption or see Washington bypass his cabinet and seek out lower level officials to provide essential services to Afghans.

The warning -- coming a day after Obama said the US government would no longer give Afghanistan a "blank check" for US aid -- turned up the pressure on Karzai to end the corruption seen as fueling the Taliban insurgency.Related article: Afghans welcome troop surge

White House spokesman Robert Gibbs told reporters that Obama talked "about the notion that it is time for a new chapter in Afghan governance" in conversations with Karzai, including a secure videoconference late Monday.

When he unveiled his new Afghan strategy in a speech on Tuesday, Obama made it clear he expected major changes from the Karzai government's status quo, and outlined several steps he wanted them to take.

"The days of providing a blank check are over," Obama said in the speech, noting that Karzai sent the "right message about moving in a new direction" when he was inaugurated last month as president for another term.

Gibbs on Wednesday took the warning a step further.

"If President Karzai is unable or unwilling to make changes in corruption or governance... we will identify people at a sub-cabinet level, at a district level that can implement the types of services and basic governance, without corruption, that Afghans need."

In her testimony to the Senate Armed Services Committee, Clinton said the administration has "real concerns about the influence of corrupt officials in the Afghan government, and we will continue to pursue them."

She also recalled Karzai's remarks at his inauguration, which she attended in Kabul, following his victory in the fraud-marred August elections.

"He (Karzai) pledged to combat corruption, improve governance, and deliver for the people of his country. His words were long in coming, but welcome. They must now be matched with action," the chief US diplomat told senators.

Gates also strove to reassure war-weary Americans that the US presence was "not open-ended" and that "it is neither necessary, nor feasible, to create a modern, centralized, Western-style Afghan nation-state, the likes of which has never been seen in that country."

He promised "a narrower focus" on routing Al-Qaeda, with "observable progress on clear objectives" but bluntly told lawmakers: "Quite frankly, I detest the phrase 'exit strategy.'"

"What is essential is -- for our national security -- is that we have two long-term partners in Afghanistan and Pakistan," he said.

On Thursday, Clinton, Mullen and Gates go before the Senate Foreign Relations Committee, while Mullen and Gates were also due to appear before the House Armed Services Committee.