Within hours of taking the oath of office, US President Donald Trump on Friday night signed an executive order aimed at trying to fulfil one of his most impassioned campaign promises; repealing the Affordable Care Act (ACA) or the Obamacare.

The multi-part executive order stated that the administration's official policy is "to seek the prompt repeal" of the ACA -- but at the same time emphasized that his administration must continue to uphold the law, media reported.

With his signature, Trump sent a powerful signal on Day One of his presidency: His top priority in the Oval Office will be dismantling the health care law that covers some 20 million Americans.

The order directs the secretary of health and human services and the heads of other agencies to use their discretion to help individuals, healthcare providers, insurers and others avoid any burdens stemming from the ACA.

The order also looks to give states more flexibility and control over their health care markets, in keeping with longstanding Republican beliefs.

Republicans have been railing against the health care law for years, using "repeal and replace Obamacare" as a political rallying cry to gin up support within the party base, media reported.  

According to Capitol Hill sources, senior Republicans are exploring multiple avenues through which to replace what gets rolled back, including measures tucked into the repeal bill, executive orders as well as administrative actions taken by Trump's nominee for Health and Human Services secretary, Tom Price.

White House Chief of Staff Reince Priebus also sent out a memo to all government agencies and departments calling for a regulatory freeze.

The action is a fairly standard move for a new administration taking over for the other party.

It effectively halts any lingering policies from the Obama administration before they can be finalised, media reported.

The move will also give Trump the ability to declare an immediate impact on the regulations that Republicans have long slammed as burdensome on businesses

Also on Friday, shortly after Trump was sworn into, the newly updated White House website said the new administration will eliminate the Climate Action Plan, the centrepiece of Obama's climate legacy that aims to cut carbon emissions from power plants.

Trump also signed papers confirming the appointments of James Mattis as Secretary of Defence and John Kelly as Secretary of Homeland Security.