Donald Trump's choice for secretary of state, former ExxonMobil head Rex Tillerson, assured senators Wednesday that the United States would continue striving for nuclear nonproliferation, a stance in conflict with some of the president-elect's pronouncements.

During his presidential campaign, Trump raised the possibility of Japan and South Korea -- both key US allies -- arming themselves with nuclear weapons.

And late last year, the president-elect revived the specter of a nuclear arms race, saying that the United States would respond in kind if any other nuclear power expanded its arsenal.

Yet Tillerson, in a confirmation hearing before the Senate Foreign Relations Committee, said that "one of the vital roles" for the State Department is "the pursuit of nuclear nonproliferation."

"We just simply cannot back away from our commitment to see a reduction in the number of these weapons on the planet," said Tillerson, who was named by Trump late last year to succeed Secretary of State John Kerry.

As the Republican-controlled Senate weighs the confirmation of Trump's cabinet nominees, most are expected to breeze through relatively unscathed.

But the nominees have provided both new clarity -- and sometimes a degree of confusion -- as to the precise policies Trump aims to pursue.

While President Barack Obama has spent much of his time in office preaching the virtues of "a world without nuclear weapons," Trump has said he is ready, if necessary, to end the decades-old pursuit by both Democratic and Republican administrations to reduce the numbers and strategic importance of nuclear weaponry.

Trump said last month in remarks reported by MSNBC that if it came to it, "Let it be an arms race... we will outmatch them at every pass and outlast them all."

He said the United States should "greatly strengthen and reinforce its nuclear capability."

But when Tillerson was asked about Trump's notion that countries such as Japan or South Korea should perhaps possess nuclear weapons, he told senators "I do not agree."

"I don't think anyone advocates for more nuclear weapons on the planet," he said.

The United States has an arsenal of some 7,000 nuclear warheads, a few hundred less than Russia possesses.

The Pentagon plans to modernize the three legs of its nuclear triad -- intercontinental missiles, submarine-launched missiles and strategic bombers -- at an estimated cost of $1 trillion over 30 years.