He's described Kim Jong-Un as a "smart cookie", Vladimir Putin as someone doing a "better job than Obama" and Egypt's Abdel Fattah el-Sisi as a "fantastic guy".

Donald Trump's praise for world leaders described by many as dictators hasn't gone unnoticed.

This week he even said that he would be "honoured" to meet the North Korean leader.

The comments came on the heels of the president extending a personal invitation to controversial Philippines President Rodrigo Duterte after a "very friendly" phone call, according to a White House statement.

Since assuming office, Duterte has led a crusade against illegal drugs that has claimed nearly 7,000 lives, largely at the hands of the police, according to Human Rights Watch.

Republican Senator John McCain (Arizona) said that he was at a loss to explain why Trump praises some of the worst dictators around the world.

In an interview with MSNBC's Morning Joe, McCain said he found Trump's remarks "very disturbing" and cautioned that the president's comments will be troubling to U.S. allies.

"I don't understand it," McCain said. "I don't think that the president appreciates the fact that when he says things like that it helps the credibility and the prestige of this really outrageous strongman," referring to the North Korean dictator Kim Jong-Un.

McCain added: "You can't praise that kind of behaviour and not raise concerns around the world."

Trump's approach is almost business-like, in that he is using the clout of America to form what is becoming his signature style in that, building personal relationships with rivals, adversaries, and even enemies can advance American interests.

"The reason that the president is building an effective coalition and is getting results around the globe in reasserting America's place is because he understands the type of diplomacy and the type of negotiating and the type of deal-making that actually gets real results for our country," White House press secretary Sean Spicer said on Monday (1 May).

But Trump's relationship with dictators is something his predecessor was keen to act on too.

In 2008, while competing against one another, a key difference between Democratic candidates Barack Obama and Hillary Clinton was Obama's willingness to talk to adversaries.

Clinton was less enthusiastic to talks with countries such as Iran, North Korea and Cuba while Obama said he would talk to them "without preconditions".

Obama revolutionised the relationships the US had with Cuba and Myanmar, but some have seen Trump's stance as taking it too far.

Ned Price, a former Obama National Security Council official, said: "President Trump's feting of autocrats and dictators, including from the Oval Office, undermines the values for which America has always stood.

"That is not only a break from past Republican and Democratic administrations, it's also an affront to America's traditional role in the world."