Defense Department officials warned a House panel Thursday that if Congress does not approve the Obama administration’s proposal for two new rounds of base closings, the Pentagon will use existing authority to close some facilities.

“Strategic and fiscal imperatives leave the department no alternative — we must close and realign military bases here in the United States,” Dorothy Robyn, deputy undersecretary of defense for installations and environment, told the House Armed Services readiness subcommittee at a hearing on the request for more Base Realignment and Closure (BRAC) rounds.

“If Congress does not authorize additional BRAC rounds . . . the department will be forced to use its existing authorities to begin to support our new defense strategy,” Robyn said.

She said that the Defense Department has more than 300,000 buildings and 2.2 billion square feet of space, which is “more than Wal-Mart.”

“We are focused on getting rid of capacity we don’t need so we can use the resources elsewhere,” Robyn said.

The proposal came under sharp bipartisan attack from panel members who criticized the administration for making the plan without providing any estimates of cost savings or other details.

“We need a lot more facts before we pass on another round of BRAC,” said Rep. J. Randy Forbes (R-Va.), chairman of the readiness subcommittee.

“I’m as concerned as other members in not knowing the cost,” said Rep. Silvestre Reyes (D-Tex.). “We’re moving in the dark with a request for two BRACs.”

Some members also argued that making further cuts ignored looming threats to U.S. security, in particular with U.S. troops still in Afghanistan and as the United States and other world powers press Iran over alleged research into nuclear weapons.

The Obama administration is “losing sight of the fact we’re still at war,” Rep. Mike D. Rogers (R-Ala.) said. “We could be in Iran in a heartbeat.”

Robyn said that the Pentagon has only begun “initial preparatory work” for a new BRAC, including making inventories of property and examining whether analytical tools for evaluating which facilities to close need to be updated.

“If we do get authorization for 2013, we can move out smartly,” she added.

Robyn said the administration is requesting two rounds because of uncertainty over how deep defense budget cuts will be. “A second round would give us a chance to adapt to things that happen subsequently,” she said.

Robyn warned that if the Pentagon is forced to make cuts outside of the BRAC process, it would be “severely constrained in what it can do to help local communities.” Unlike under BRAC, communities would have no role in determining the fate of property and would have to pay fair-market value for the land, she said.

“Communities would have to fend for themselves to a much larger degree,” Robyn said.

Many in Congress are still licking their wounds from the 2005 BRAC, the largest and costliest round ever, which resulted in 190 closures and realignments.

A Defense Department budget report submitted to Congress last month showed that the total one-time cost for implementing the last round of BRAC was $35.1 billion. This cost was about 67 percent higher than the $21 billion that had been estimated by the Defense Base Closure and Realignment Commission in September 2005.

Robyn acknowledged that the 2005 BRAC round will not break even until 2018. But she said that the last BRAC round “is not the right comparison” to the proposed new rounds because it “was not about savings.”

The argument was supported by a Government Accountability Office report issued Thursday, which found that the results of the 2005 BRAC were heavily influenced by then-Defense Secretary Donald H. Rumsfeld’s goals of transforming the military and fostering cohesiveness among the military services.