The decision, coming after a 2013 U.S. Supreme Court ruling that struck down a federal ban against same-sex marriage, allows the Social Security Administration (SSA) to determine the eligibility of married gay applicants to Medicare, the federal government's healthcare program for the elderly and the disabled.

The U.S. Centers for Medicare and Medicaid Services (CMS)said the government has begun the enrollment process for some same-sex spouses while handling requests for special enrollment periods from others. CMS oversees the $635 billion Medicare program. But SSA determines eligibility.

"If you're in, or are a surviving spouse of, a same-sex marriage, we encourage you to apply for Medicare if you think you might be eligible," CMS said in a web posting.