The US Supreme Court on Tuesday handed President Donald Trump a victory on his policy barring many transgender people from the military, lifting lower court rulings that had blocked it on constitutional grounds from going into effect.

The decision, with the court's five conservative justices prevailing over its four liberals, granted the Trump administration's request to put on hold injunctions issued by federal judges against enforcement of the policy while a challenge to its legality continues in lower courts.

The court did not resolve the underlying question of the legality of the Republican president's plan, which reversed the landmark 2016 policy of his Democratic predecessor Barack Obama to let transgender people for the first time serve openly in the armed forces and receive medical care to transition genders.

But in lifting the injunctions, the court signalled it would probably decide in favour on the administration when it is eventually asked to rule on the merits of the challenge brought by transgender people already in the military or hoping to join. The plaintiffs argued that the policy violated the US Constitution's guarantee of equal protection under the law.

The justices declined the administration's request to immediately take up the fight over the policy's legality even before a California-based federal appeals court that often has been criticised by Trump rules on the matter.

Attorneys for the plaintiffs condemned the court's action and said some current troops could face discharge. Various injunctions had allowed new transgender troops to join the military as of January 1, 2018, in addition to the estimated thousands already serving. The US military allowed gay troops to serve openly for the first time starting in 2011 under Obama.

'For more than 30 months, transgender troops have been serving our country openly with valour and distinction, but now the rug has been ripped out from under them, once again,' said Peter Renn, an attorney for Lambda Legal, which represents some of the plaintiffs.

House of Representatives Speaker Nancy Pelosi, a Democrat, said Trump's 'ban on trans Americans serving in our nation's military was purpose-built to humiliate brave men & women seeking to serve their country'.

The Department of Defense praised the court's action.

'As always, we treat all transgender persons with respect and dignity. DoD's proposed policy is NOT a ban on service by transgender persons,' the Pentagon said in a statement, adding that the policy was based on 'professional military judgment and will ensure that the US Armed Forces remain the most lethal and combat effective fighting force in the world'.

Trump, whose administration also has taken other steps to limit the rights of transgender Americans, in 2017 announced a plan to ban transgender people from the military. Trump cited the 'tremendous medical costs and disruption' of having transgender troops.

In March 2018, Trump backed a revised policy crafted by then-Defense Secretary Jim Mattis to ban transgender people who seek or have undergone gender transition steps. It also would ban under certain circumstances transgender people who experience gender dysphoria, a condition the American Psychiatric Association defines as clinically significant distress due to 'a conflict between a person's physical or assigned gender' and the individual's gender identity.

Trump got a boost on January 4 when a federal appeals court overturned an injunction issued by a judge in the US capital. A fourth injunction issued by a judge in Maryland will have to be lifted before Trump's policy can be implemented, but the administration said the justices' action applies to that one, too.

A 2016 Pentagon-commissioned study found that any impact on cost or military readiness from having transgender troops would be marginal. It estimated there were around 2,450 transgender troops at the time. Not all transgender people experience gender dysphoria, according to the American Psychiatric Association, which opposes a transgender military ban.

The new policy lets people diagnosed with gender dysphoria after entering the military continue to serve only if they are willing to do so in their 'biological sex'. The administration has said 937 active-duty service members have been diagnosed with gender dysphoria since 2016.

Mattis announced his resignation in December over other differences with Trump.

Trump's administration has rescinded federal guidance protecting transgender students in public schools concerning bathroom access, while the Justice Department has argued that a federal law against workplace discrimination on the basis of sex does not cover transgender employees.

The Supreme Court in 2017 sidestepped a major ruling in another transgender rights case when it cancelled arguments in a bathroom access dispute involving a Virginia high school student after the administration reversed the federal guidance.