Rex Tillerson, the former chairman and chief executive of Exxon Mobil, has been sworn in as President Donald Trump’s secretary of state.

The Texas native, 64, was cleared for full Senate approval in a 56-43 vote.

The vote came after Senate Republicans changed the rules in order to approve Mr Trump’s nominees for health and treasury, despite a Democratic boycott.

Mr Tillerson, who has never held political office, faced intense scrutiny over his ties to Russia.

As Mr Tillerson was sworn by the vice-president in the Oval Office, the president told him: “Your whole life has prepared you for this moment.”

Mr Trump – who had just returned from a Delaware air base where he met the returning remains of a Navy SEAL killed in a weekend raid in Yemen, the first known US combat casualty since he took office – was sombre and subdued in tone.

America’s new top diplomat thanked him and said: “As I serve this president, I will always represent the interests of the American people at all times.”

The former oil chief forged multibillion-dollar deals with Russia’s state oil company, Rosneft, and was awarded the Order of Friendship by the Kremlin in 2013.

In his Senate confirmation hearing, he admitted that the West had reason to be alarmed by Russian aggression, but he refused under questioning to label Mr Putin a war criminal.

As secretary of state, Mr Tillerson will handle relations with countries including China and Russia and negotiate matters such as climate change and human rights, on behalf of President Trump.