The federal judge who threw Kentucky county clerk Kim Davis into jail last year for refusing to issue marriage licences to same-sex couples said she had not interfered with the process since her release and that licences with her name removed should still be valid.

US District Court Judge David Bunning ruled late on Tuesday that Rowan County's Davis, who spent five days in jail last September for her stand on marriage licences for gay couples, had not interfered with her deputies' issuing the licences.

When she returned to work, Davis removed her name, title and personal authorisation from the licences.

"There was every reason to believe" the forms she altered would be deemed valid by state officials, Bunning said, adding that the court would continue to monitor Davis and her office.

Citing her beliefs as an Apostolic Christian, Davis, 50, drew national attention for her refusal to issue any licences after a US Supreme Court ruling in June made gay marriage legal across the nation. Homosexual and heterosexual couples subsequently sued her, and her jailing drew international attention and demonstrations from both sides of the issue.

The American Civil Liberties Union, which represents the couples suing Davis, had said she made material changes to the marriage forms, which put her out of compliance with Bunning's orders to issue the licences as part of her job. ACLU lawyers had asked for Davis to be held in contempt and her office to be put in receivership.

ACLU lawyer Ria Mar said Bunning's ruling could not be appealed, and it is for a state court to decide the validity of the altered licences.

"The bottom line is that all loving couples in Rowan County can now receive marriage licences, and we'll continue to fight to ensure that remains the case," she said.

Davis's lawyers had said she had taken reasonable steps to comply with the judge's orders and should not face further contempt citations.

"From the beginning we have said the ACLU is not interested in marriage licences," said Davis' lawyer Mat Staver. "They want Kim Davis' scalp."

Kentucky's attorney general previously said the altered licences were valid. In December, newly elected Republican Governor Matt Blevin ordered county clerks' names removed from marriage licence forms to honour people's religious beliefs.