Chairman of the Joint Chiefs of Staff Adm. Michael Mullen (R) and Under Secretary of Defense for testify during a hearing before the House Armed Services Committeeon Capitol Hill. (Alex Wong/GETTY IMAGES)

The top U.S. military officer told lawmakers Thursday that he supports President Obama’s plan to draw down forces in Afghanistan, but he said Obama’s decisions were “more aggressive and incur more risk that I was originally prepared to accept.”

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told the House Armed Services Committee that Obama’s decisions also have the support of Gen. David H. Petraeus, the top U.S. commander in Afghanistan, and Gen. James N. Mattis, head of the U.S. Central Command.

In a separate hearing before the Senate Foreign Relations Committee, Secretary of State Hillary Rodham Clinton said she agrees 100 percent with Obama’s plan, which she said took into account widely divergent views among his national security team and “pressing” domestic needs.

Clinton came under skeptical questioning from both parties over the level of U.S. spending and sacrifice in Afghanistan. But she asserted that despite the “many failings” of the government of President Hamid Karzai and the “many challenges that remain” in Afghanistan, “life is better for most Afghans,” and there is broad evidence of progress. “We are, and should be, encouraged by what we have accomplished,” Clinton said.

“There is no jumping ship here. Quite the contrary,” Mullen told the House committee. “We will have at our disposal the great bulk of the surge forces throughout this — and most of the next — fighting season.”

“Let me be candid, however,” Mullen said. “No commander ever wants to sacrifice fighting power in the middle of a war. And no decision to demand that sacrifice is without risk.”

Mullen said that despite his initial skepticism, he was comfortable with the president’s decision. “Only the president, in the end, can really determine the acceptable level of risk we must take. I believe he has done so.”

Gen. David H. Petraeus, the top U.S. commander in Afghanistan, said in separate Senate testimony that Obama decided to pursue a “more aggressive” drawdown than the military had recommended in Afghanistan.

“That is understandable in the sense that there are broader considerations beyond those of a military commander,” Petraeus told the Senate Select Committee on Intelligence, which is considering Obama’s nomination of him to be the next director of the CIA. “The fact is there has never been a commander in military history” who got everything he wanted, the general added. He said he agreed with Mullen that the risks associated with the faster drawdown “are at the margin.”

“Ultimately the decision was made,” Petraeus said. “Obviously I support that and will do everything I can in my remaining time as commander to implement it.” If confirmed as CIA director, he said, he would “do the same from that position as well.”

Obama’s military commanders had requested that the bulk of the surge forces remain in Afghanistan through the end of next year, giving them another full fighting season at nearly current strength in addition to the one underway. But the president also faced growing calls from Vice President Biden and other advisers in the White House for all of the surge forces to return as soon as the end of this year.

The president Wednesday night announced that he would chart a middle course, bringing home 10,000 U.S. troops by the end of the year and 23,000 more by next summer.

Mullen, along with Michèle Flournoy, the undersecretary of defense for policy, emphasized the continued growth of the Afghan security forces and said that only the Afghans could ultimately achieve lasting gains in their country.

“This announcement in no way marks a change in American policy or strategy in Afghanistan,” Flournoy said.

Mullen noted that, with the growth of the Afghan forces, the Taliban could face more combined force – in terms of numbers of troops – in 2012 than it did this year.

In a statement issued shortly after the president’s speech, Defense Secretary Robert M. Gates said he supported the decision “because it provides our commanders with enough resources, time and, perhaps most importantly, flexibility to bring the surge to a successful conclusion.”

In response a question at the Senate Foreign Relations Committee hearing, Clinton declined to specify what Petraeus recommended. But she added: “It would be totally understandable that a military commander would want as many troops for as long as he could get them.” The commander, though “also knows that what he wants is just part of the overall decision matrix” and that there are “other factors at work.”

Clinton said various advisers recommended that troops get “out now, out at the end of the year, out by the beginning of the year” and that “what the president decided was to get through the next fighting season, in effect.”

The logic behind the decision to withdraw 33,000 U.S. troops by the end of next summer, Clinton said, was to “recover the surge” that Obama began last year in about the same length of time — 18 months — that it took to implement it.

As Obama considered his options, there were “lots of competing opinions coming at him from all sides,” Clinton said.

While some lawmakers endorsed the president’s plan, others expressed concern that it would undermine the gains the U.S. military has achieved.

“The president’s decision could jeopardize the hard-won gains our troops and allies have made over the past 18 months and potentially the safety of the remaining forces,” said Howard P. “Buck” McKeon (R-Calif.), chairman of the House Armed Services Committee. “This announcement also puts at risk a negotiated settlement with reconcilable elements of the Taliban who will now believe they can wait out the departure of U.S. forces and return to their strongholds.”

Mullen declined to say how many of the troops coming home before the end of the year would be combat units. He said the specific breakdowns have not yet been determined and that commanders in Afghanistan would ultimately make the decisions based on conditions on the ground.

Sen. Richard G. Lugar (Ind.), the top Republican on the Senate Foreign Relations Committee, told Clinton that “no rational review” of U.S. strategic interests could justify committing 100,000 troops and $120 billion a year to Afghanistan at this point, especially when terrorist threats in other countries — notably Yemen, Pakistan and Somalia — pose greater dangers to the United States.

Sen. Robert Menendez (D-N.J.) agreed, noting that al-Qaeda leader Osama bin Laden is now dead and that his organization is estimated to have fewer than 100 fighters in Afghanistan.

Moreover, he said, the United States will have spent $38 billion by the next fiscal year to “prop up and train” a 290,000-strong Afghan government force to fight an estimated 20,000 Taliban insurgents in a country that soaks up billions more in U.S. development assistance and whose president “talks about us an an occupying force.”

Staff writers Karen DeYoung and Felicia Sonmez contributed to this report.