WASHINGTON, D.C. (AP) — Homeland Security officials are pushing recalcitrant states to adopt stricter driver’s license standards to end a standoff that could disrupt domestic air travel.

States have less than a month to send a letter to the Homeland Security Department seeking an extension to comply with the Real ID law passed following the 2001 terror attacks. Some states have resisted, saying it is costly, impractical and an invasion of privacy.

Four states — Maine, Montana, New Hampshire and South Carolina — have yet to seek an extension.

Homeland Security Secretary Michael Chertoff argues that the law fixes a critical gap in security identified by the commission that investigated the 9/11 attacks: the ease of obtaining government-issued ID. It will also hinder would-be con artists and illegal immigrants, he said.

Real ID-compliant driver’s licenses would have several layers of new security features to prevent forgery. They would also be issued after a number of ID checks, including verification of birth certificates, Social Security numbers and immigration status. Officials acknowledge it will take years to phase in all the different security measures.

To bring the states in line, Chertoff warned that any state that does not seek an extension by the end of March will find that, come May, their residents will not be able to use their licenses to board domestic flights.

Chertoff’s assistant secretary, Stewart Baker, sent letters to several governors Monday reminding them of the looming deadline, and urging the holdouts to seek an extension.

In recent years, 17 states passed legislation or resolutions opposing Real ID, but now only a handful appear willing to challenge the government publicly.

Officials in Maine and Montana insisted Monday they would not seek an extension. A spokesman for South Carolina’s governor said he was still considering it. New Hampshire passed a law last year prohibiting the state from participating in the Real ID program, and Gov. John Lynch wrote Chertoff last week asking him not to impose the requirements on New Hampshire citizens.

A fifth state, Delaware, has sent a letter asking for an extension, but DHS officials are still weighing whether the wording of the letter legally adds up to an extension request.

If the states do not seek an extention by March 31, their residents will be subjected to secondary screening by security workers before boarding any domestic flight beginning May 11.

‘‘We’re not going to buckle under here,’’ said Montana Gov. Brian Schweitzer. ‘‘My guess is the people of Montana would be proud to walk through that line.’’

Schweitzer called the Real ID proposal a bureaucratic boondoggle that will cost his state a fortune and give a false sense of security without actually making ID more reliable. He has sought to rally opposition to Real ID, but the vast majority of states have decided not to test whether Washington is bluffing.

As the high-stakes game of chicken continues, federal authorities are not publicly saying whether seeking an extension actually counts as complying with the law. In his recent letters, Baker said only that the 45 states that have sought extensions are ‘‘on track toward improved security.’’