WASHINGTON – 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. It was passed after 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 Sept. 11 attacks: the ease of obtaining government-issued ID. It also will 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 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 security measures.

To bring the states in line, Chertoff warned that any state that does not seek an extension by the end of March would find that, come May, its 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.

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 residents.

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