A message left with Kissinger’s consulting company, Kissinger Associates, asking whether the firm has a financial relationship with Exxon, wasn’t immediately answered.

Weeks before he takes office on January 20, Trump has injected an air of unpredictability into US relations with both allies and adversaries. In addition to saying he wants friendlier ties with Russia, Trump has slammed China over currency and trade, had an unprecedented call with Taiwan’s leader, praised the Philippine president’s violent war on drugs and promised to visit Pakistan, a sign of validation US President Barack Obama never bestowed on Islamabad.

That’s a lot of material for Kissinger, who brokered the US rapprochement with China as Richard Nixon’s secretary of state more than four decades ago and has advised US presidents ever since. With Trump heading to the White House next month, Kissinger continues to fulfil a role as unofficial adviser to the world’s most powerful leaders, visiting both Trump in his New York offices and Chinese President Xi Jinping in Beijing since the November 8 election.

Kissinger chided Trump for making Taiwan a key issue at the beginning of his dialogue with China, by taking a protocol-breaking phone call with the island’s president, Tsai Ing-wen, this month. He called the move “unwise” and “not the most efficient.” However, he said he is “hopeful, optimistic, and confident” that Trump will uphold the decades-old “One-China” policy and avoid diplomatic or formal relations with an island that China considers part of its territory.

“One should give these things a little time, the new administration isn’t in office yet,” he said. “Every president of the US since 1971 and both parties have accepted this framework and once that framework is studied, I do not expect it to be overturned.”

Kissinger didn’t say what advice he gave the Chinese during his most recent visit, but the government’s official response to Trump’s Taiwan call was measured. While Beijing lodged a “solemn representation” - a mild form of diplomatic protest - and urged US authorities to adhere to the so-called One-China principle, it stopped short of criticising Trump.

“Some would argue that Trump’s out-of-the-line talk and behaviour was meant to poke at China, and that the Chinese should retaliate,” the People’s Daily, the Communist Party’s flagship newspaper, said in a front-page story this month. “Otherwise he might think China is a soft persimmon, easy to pinch.” But the country “doesn’t throw a tantrum to strive for only temporary superiority, or to gain immediate gratification.”