Thomas DiNanno, the under secretary of state for arms control and international security, repeated that it would not be an atmospheric test of the sort seen at the start of the nuclear era.

"We are still assessing," DiNanno told the Senate Foreign Relations Committee.

"We have made no decision specifically on how or what any testing program would look like."

Trump said in October that the United States would resume nuclear testing for the first time since 1992.

Trump later allowed the expiration of New START, the last remaining treaty with Russia that limited deployment of nuclear warheads, as he called for a new agreement that also includes China, whose arsenal is much smaller than those of the other two powers but quickly growing.

Another senior official, Christopher Yeaw, said last month that renewed US tests would be low-yield to match what the United States alleges China and Russia have secretly carried out.

Asked if the United States could instead carry out a larger detonation, DiNanno said, "I've heard no discussion of any atmospheric testing whatsoever."

China has denied the allegations of secret testing, accusing the United States of creating an excuse to pressure it into a new nuclear agreement.

The US talk of new nuclear testing comes as the United States and Israel attack Iran in part over allegations that the cleric-run state was building a nuclear weapon, a prospect denied by Tehran and which most experts said was not imminent.

The United States is the only country to have used nuclear weapons in war, destroying the Japanese cities of Hiroshima and Nagasaki in 1945, and Israel has a widely discussed but never officially acknowledged nuclear program.