The world could risk a nuclear war if U.S. President-elect Donald Trump did not control his "fierce" temper, the former head of MI6 has said. Sir John Sawers said Trump needs to keep a cooler head when dealing with his perception that he has been insulted.

Sawers said Friday that there was a huge risk of a "great military clash" if Russia or China misread a situation or the new president "overreacts to something." Trump's biggest test will be forming good relationships with Russian President Vladimir Putin and China's Xi Jinping, in order to deal with the potential North Korean nuclear threat.

"We're getting back into a world which is quite dangerous, and I think that is the biggest threat," Sawers told BBC Radio 4's "World At One."

"It's got worse over the last five or six years partly because, frankly, one of the failures of President Obama's administration is managing its relationships with Russia," the former spy chief said, adding that the "worse-case scenario" would be those relationships turning more confrontational.

"I don't think Donald Trump quite yet knows what the pressures will be on him when he becomes president," Sawers said. "We've seen that when he feels slighted, when he feels criticised, he reacts quite fiercely ... If you translate that into global affairs it could have consequences which are dangerous for everyone."

In fact, the question of war under a Trump presidency becomes imminent when discussing the use of nuclear weapons as the business mogul had once asked a foreign policy expert why U.S. cannot use nuclear weapons, an American talk show host claimed. President Barack Obama has also repeatedly stated that he would not trust Trump with the nuclear launch codes for U.S. intercontinental nuclear ballistic missiles given the latter’s temperament.

Obama mocked Trump last week for having his Twitter account being used by his aides, saying: “If somebody can’t handle a Twitter account, they can’t handle the nuclear codes.” Obama said Trump would be “dangerous” as commander-in-chief.

According to experts, Trump’s volatile temperament could be exploited by U.S. adversaries to lure the country into unnecessary conflict.