With his inauguration fast approaching, US president-elect Donald Trump has been actively engaging in foreign policy as if he were already in office.

Many of his reactions and comments on US foreign policy delivered over Twitter, were direct and immediate. And several of them overshadowed and undermined President Barack Obama's attempts to put the finishing touches on his own foreign policy legacy.

Trump's sharpest departure from the Obama administration came when he publicly demanded the United States veto a resolution in the UN Security Council condemning settlement expansions in the West Bank.

The US allowed the resolution to pass by abstaining.

When Israeli Prime Minister Benjamin Netanyahu complained, saying Israelis didn't need US Secretary of State John Kerry's "lecture" on Israel's expansion of its settlements, Trump told the country to "stay strong."

When Russian President Vladimir Putin opted not to retaliate by expelling US diplomats over a hacking row with the White House, Trump called it a "great move," adding "I always knew he was very smart!"

Trump also used Twitter to spout his opinions on the US' nuclear capacity, saying it should be expanded, contradicting current doctrine.

And mixing it up with China, Trump said Beijing could keep a US underwater drone it wanted to return after confiscating it.

The apparent frustration of the Obama administration showed through when spokesman Ben Rhodes responded to Trump's activity, saying "There is only one president for the US till January 20. This is a long, bipartisan US policy," he said.

The transfer of power after an election can be a delicate matter in any country, but when it's a superpower other countries look on with particular interest.

In the United States there aren't any laws governing exactly how a transition should proceed, which has served to complicate the situation.

Trump isn't following the unwritten rules either, putting the country into a somewhat bizarre situation. Meanwhile, Obama administration officials are fighting to prevent his legacy from being battered by the Trump wrecking ball.

Outwardly, the transfer of power has appeared to be going smoothly, with Trump making references to how friendly Obama has been. Since their tense, but polite, White House meeting two days after the election, Obama hasn't tired of stressing how important a peaceful transition is to him.

He said the fact that there is only one president is one of the vital functions of a democracy - as important as civilisation and tolerance and a commitment to reason, facts and analysis.

But Trump isn't someone who sits and waits, spokesman Sean Spicer said, even in the face of criticism of his willingness to jump the gun.

"In some ways, Trump is neutering the Obama administration," said Douglas G Brinkley, a professor of history and a presidential historian at Rice University in Houston.

Brinkley told the New York Times that while they have avoided personally attacking each other, behind the scenes they're working to undermine each other.

Other transitions have been more civilised.

Bill Clinton said in 1992 that he wanted to assure the world of the fundamental continuation of American foreign policy and after the 2000 election president-elect George W Bush refused to respond to Israel and North Korea, saying the country had to speak with "one voice."

Obama said the US would maintain these values ??and maintain these ideals "as long as I am president."

But Trump seemed determined to muddy the political landscape once again on New Year's Eve with his parting shot of 2016.

"Happy New Year to all, including to my many enemies and those who have fought me and lost so badly they just don't know what to do. Love!"