WASHINGTON — The Bush administration plans to announce that it will ease into — but not abandon — tougher passport rules planned for U.S. border crossings next year, according to congressional aides briefed on the matter.

The Homeland Security Department, responding to a torrent of complaints about delays in passport applications that have hampered summer travel plans, will alter its requirements for Americans at land and sea crossings beginning in January.

The rule would affect U.S. citizens traveling to Canada, Mexico, the Caribbean and Bermuda.

Under a post-Sept. 11 security law passed by Congress, U.S. citizens are required to show passports at such land and sea crossings beginning in 2008, but Homeland Security officials privately told legislative staffers late yesterday that the rule will, at least at the beginning, require proof of citizenship — meaning a passport or a birth certificate would suffice. That requirement could last until spring, according to those familiar with the plan.

Congressional aides briefed on the matter spoke on condition of anonymity because the plan will not be made public until today.

The new rule will also end a little-known practice of "oral declarations" — instances in which a person crossing the border can make a statement declaring his or her citizenship without providing any documentation, aides said.

At a Senate hearing yesterday, lawmakers peppered Maura Harty, assistant secretary of state for consular affairs, about the passport rules. Harty pledged the government's plan for land and sea crossings "will be very flexible," but she did not elaborate.

Senators remained skeptical, citing the backlog in passport applications.

"This is just another example of ineptness that absolutely destroys Americans' — including mine — confidence in the federal government doing anything right and competent," said Sen. David Vitter, R-La.

As recently as last week, Homeland Security officials insisted they were not backing off the January deadline, but they have been under intense pressure since a similar passport requirement — begun five months ago for air travel to Canada, Mexico, the Caribbean and Bermuda — caused major headaches.

The new air travel rule caused a flood of passport applications, leading to a backlog at the State Department processing centers that postponed or ruined the summer travel plans of thousands of Americans.

Complaints from the public and from Congress about those delays forced the Bush administration to suspend the air travel passport requirement.

Officials announced this month that those flying to Canada, Mexico, the Caribbean and Bermuda may now do so until September with an identification card like a driver's license and a printout from a State Department Web site showing they have applied and are still waiting for a U.S. passport.

At the hearing of a Senate Foreign Relations subcommittee, lawmakers peppered Harty with questions about how they plan to solve the passport backlog.

"We want to know who's accountable and why this mess has happened," said Sen. Bill Nelson, D-Fla., who chaired the hearing.

Harty said the delay came from a miscalculation on the size of the surge in passport demand and partly blamed Hurricane Katrina, which reduced the capacity of the passport office in New Orleans.

"I take responsibility for it," Harty said.