As the Obama administration reorients its military strategy toward Asia and the vital maritime trade routes in the Pacific, the bulk of the responsibility will fall on the Navy, which was largely sidelined during the land wars of the last decade.

But the Navy will have to perform its mission in Asia with fewer ships in coming years than it had anticipated. Under President Obama’s proposed defense budget, the Navy will retire nine ships early and cut or delay the purchases of 16 others over the next five years.

The Navy had long planned to increase the size of its 285-ship fleet to 313 vessels by 2020, but under Obama’s budget it will fall far short. Under the new plan, the fleet will remain at 285 ships over the next five years.

The Navy is hoping to expand to a 300-ship fleet by 2019, but that’s only if the service doesn’t get hit with additional spending cuts, an optimistic scenario.

The changes have prompted criticism from Congress, where some lawmakers have pressed Defense Secretary Leon E. Panetta and Navy officials on how they can carry out Obama’s new strategy for Asia with fewer ships and other resources than they had been counting on.

“Cuts to our naval capabilities such as these, without a plan to compensate for them, only put our goals in the Pacific region at greater risk,” Sen. John McCain (R-Ariz.), the vice chairman of the Senate Armed Services Committee, said at a Feb. 9 confirmation hearing for Adm. Samuel J. Locklear III, whom Obama has nominated to take over the U.S. Pacific Command.

Navy leaders play down the cuts and say they will be able to carry out the president’s strategy with the same number of ships they have now. They noted that the Navy will maintain all 11 of its aircraft carrier groups — the crown jewels of the fleet.

Navy Secretary Ray Mabus said most of the ships that will be retired early are old cruisers that either lacked ballistic missile capabilities or that needed expensive repairs. Most of the ships that will be built later than previously planned, he added, are smaller support vessels.

“We’re losing some ships that are not as capable as the new ships coming in,” Mabus said in an interview Wednesday in his Pentagon office, a day before he is scheduled to testify on Capitol Hill about the Navy’s proposed budget. “We’ve got enough to meet the war plans with what we’ve got under contract.”

Mabus said the Navy was altering deployment plans and embracing some creative manpower arrangements that will enable the service to keep ships at sea longer, especially in Asia.

For example, the Navy is planning to base four new Littoral Combat Ships in Singapore, Mabus said. The warships are the most modern in the Navy’s fleet and can be outfitted for a variety of missions, from combating piracy to tracking submarines and carrying out special operations missions. They’re designed to operate in coastal waters and travel at a top speed of more than 40 knots.

Navy officials said they expected all four ships to deploy to Singapore, amid some of the busiest shipping lanes in the world, by 2016. Defense officials said they are still finalizing details with the government of Singapore, whose approval is needed.

By basing the ships in Singapore, the Navy would eliminate the need for lengthy trans-Pacific crossings on each deployment. In another change that will allow for longer missions at sea, three crews will be assigned to every two ships on a rotating basis, flying back and forth from the United States.

“It enables us to have a far more persistent presence all across the western Pacific,” Mabus said. Such an approach, he said, reduces the need to send extra ships to Asia if a crisis erupts.

“If something happens, we’re not escalating the situation sending ships in, because we’re already there,” Mabus said. “We can respond very quickly and we can respond in a way that doesn’t heighten tensions.”

The Navy has about 50 ships deployed to the western Pacific region — about half of its total number of ships that are at sea on any given day around the world. Approximately one-third are in the Middle East, with most of the remainder in Europe.

Adm. Jonathan Greenert, the chief of naval operations, said there are no plans to change those numbers in Asia, even with the Obama administration’s renewed emphasis on that region.

“My first assessment is we’re in good shape in the Navy where we stand in the western Pacific,” Greenert said last month at the Center for a New American Security, a Washington think tank with close ties to the Obama administration. “This is about the right proportion I see for the near term.”

At the same time, the Navy will adjust its presence in the region by spending more time in Southeast Asia, where China has alarmed several countries by aggressively staking claims to disputed territory in the energy-rich South China Sea.

Besides adding four ships in Singapore, the Pentagon will soon station a rotating force of Marines at an Australian base in Darwin. The Obama administration is also talking with the Philippines about expanding the U.S. military presence there, including the possibility of operating ships from the former Subic Bay naval base.