The chief of the US Coast Guard has said he "will not break faith" with transgender personnel after Donald Trump said they would be banned from serving in the military.

The US Coast Guard commandant, Admiral Paul Zukunft, offered support for transgender.

Speaking at a Center for Strategic and International Studies event in Washington, Admiral Zukunf said the first thing the Coast Guard did when it saw the US President announce the ban on Twitter was reach out to all 13 members who identify as transgender.

He did not say what the Coast Guard, which is part of the Department of Homeland Security, would do if the ban was implemented.

Donald Trump bans transgender people from the military

Admiral Zukunft said he contacted Lieutenant Taylor Miller, the first openly transitioning officer in the Coast Guard, who was featured in a Washington Post article.

“If you read that story, Taylor’s family has disowned her. ... And I told Taylor: 'I will not turn my back,'" The Hill reported him as saying.

Addressing the service members, he added: "We have made an investment in you, and you have made an investment in the Coast Guard and I will not break the faith.

“And so that was the commitment to our people right now. Very small numbers, but all of them are doing meaningful Coast Guard work today.”

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The Admiral said he had contacted John Kelly, the former Homeland Security Secretary who became White House chief of staff on Monday, who then reached out to Defence Secretary James Mattis.

"We've stood up a tiger team of our JAG officers," he said, referring to the Judge Advocate General's Corps, the Coast Guard's legal branch.

Mr Trump announced his ban on transgender military service last week on Twitter.

The US President said he would "not accept or allow" transgender people to serve "in any capacity."

Fifty-six retired US generals, admirals and other senior officers have voiced opposition to Mr Trump's ban, saying it "would cause significant disruptions" and deprive the military of talent or force members to live a lie.

In an open letter, retired officers said the move would "degrade readiness" even more than the 'don't ask, don't tell' policy implemented in 1994 under President Bill Clinton that lifted a ban on gay people serving in the military but called for them to be circumspect about their sexuality.

"Patriotic transgender Americans who are serving - and who want to serve - must not be dismissed, deprived of medically necessary health care, or forced to compromise their integrity or hide their identity," the former officers wrote.