WASHINGTON (AFP) - US President Donald Trump's love-hate attitude towards predecessor Barack Obama goes back a decade, a volatile mixture of admiration and scurrilous disrespect that helped launch the provocative billionaire's own political career.

Trump was a main purveyor of the "birther" conspiracy theory that the nation's first African-American president was not born in the United States - and yet went out of his way to praise him after November's bitterly contested election, even declaring last month that Obama "likes me" and "I like him".

But the new president's explosive and unsubstantiated charge this weekend that Obama tapped his phones, in a scandal he likened to "Nixon/Watergate" in a Twitter broadside, marked a brutal return to the belligerence of recent years.

Trump's up-and-down stance towards Obama goes back to 2006, when the real estate tycoon was rising as a reality television star.

That December he told The New York Times that it was "not a good sign" that Obama, then a US senator, was involved in a shady deal with a controversial Chicago businessman.

"But he's got some wonderful qualities," Trump said of Obama.

For the next several months, as Obama mapped out his remarkable presidential run, Trump offered glowing praise, in March 2007 branding him "a star".

The positive reviews continued after Obama won the 2008 election. But by April 2009 Trump was voicing concern about health care spending, inflation and taxes. One year later he was wavering on Obamacare, telling CNN "I'm really torn" .

Trump's admiration soon flipped to scorn, and he became a chief proponent of the conspiracy theory - fuelled by racist undertones - that Obama was not born in the United States.

"Why doesn't he show his birth certificate?" Trump asked on ABC's The View show on March 23, 2011.

The Obama White House obliged a few weeks later - and the president went on to skewer Trump at the White House Correspondents Dinner that April.

"No one is prouder to put this birth certificate matter to rest than the Donald," Obama said, as Trump sat in the audience with a stiff smile on his face.

"And that's because he can finally get back to focusing on the issues that matter - like, did we fake the moon landing?" Writer Adam Gopnik, seated nearby that evening, suggested in a New Yorker piece in late 2015 that the thin-skinned Trump decided then and there to seek his revenge.

"On that night, Trump's own sense of public humiliation became so overwhelming that he decided, perhaps at first unconsciously, that he would, somehow, get his own back - perhaps even pursue the presidency after all, no matter how nihilistically or absurdly, and redeem himself," Gopnik wrote.

Trump dismissed the speculation that the abuse by Obama and comedian Seth Myers that night spurred his presidential bid.

But Trump continued to gin up the birther conspiracy, even after it had been widely discredited.

"An 'extremely credible source' has called my office and told me that @BarackObama's birth certificate is a fraud," Trump tweeted in August 2012, at the height of the presidential race between Obama and Republican Mitt Romney.

Obama pushed back that October, joking on NBC's Tonight Show that the rivalry with Trump "dates back to when we were growing up together in Kenya". By 2016, with Trump having won the Republican nomination, the animosity was on full view - as Obama revved up his own attacks on the maverick candidate.

Trump "doesn't appear to have basic knowledge around critical issues in Europe, in the Middle East, in Asia, (which) means that he's woefully unprepared to do this job," Obama said on Aug 2.

Trump fired back on Twitter, saying Obama "will go down as perhaps the worst president in the history of the United States". Last September, bowing to mounting pressure, Trump finally acknowledged that Obama was born in the United States.

And two days after his shock victory, Trump met with Obama and described the outgoing president as "a very good man", saying it was a "great honour" to meet with him at the White House.

But since taking office - with an inaugural address that painted the Obama years as a dark era in which "there was little to celebrate" - Trump's tone towards his predecessor has hardened.

Obama meanwhile broke his silence, 10 days after leaving the Oval Office, to voice his support for the wave of protests against Trump's travel ban targeting refugees and citizens of Muslim-majority nations.

And any remaining doubts about the hostility between the 44th and 45th presidents were laid to rest by Trump's wiretap allegations on Saturday - which Obama rejected via a spokesman as "simply false".