HALIFAX, Nova Scotia — The top officer at US Strategic Command said over the weekend an order from President Trump or any of his successors to launch nuclear weapons can be refused if that order is determined to be illegal.

Air Force General John Hyten, head of Strategic Command, told a panel at the Halifax International Security Forum on Saturday that he and Trump have had conversations about such a scenario and that he would tell Trump he couldn’t carry out an illegal strike.

In the event that Trump decided to launch a nuclear attack, Hyten said he would give strike options that are legal. He did not elaborate on what the legal parameters would be.

The comments come amid the threat of nuclear attack from North Korea, and Trump’s critics question his temperament. There are concerns, primarily among congressional Democrats, that he may be inciting a war with North Korea.

During testimony before the Senate Foreign Relations Committee last week, retired General Robert Kehler who served as the head of Strategic Command from 2011 to 2013, also said the US armed forces are obligated to follow legal orders.

Hyten said he has talked this over with Trump. ‘‘We think about these things a lot,’’ the general said. “When you have this responsibility, how do you not think about it?’’ he said.

During his testimony, Kehler conceded that a general who refused an attack order would “be in a very interesting constitutional situation.’’

Brian McKeon, a senior policy adviser in the Pentagon during the Obama administration, said a president’s first recourse would be to tell the defense secretary to order the reluctant commander to execute the launch order.

‘‘And then, if the commander still resisted,’’ he said, ‘‘you either get a new secretary of defense or get a new commander.’’

Bruce Blair, a former nuclear missile launch officer and cofounder of the Global Zero group that advocates eliminating nuclear weapons, said the Strategic Command chief could be bypassed by a president.

A president can transmit his nuclear attack order directly to a Pentagon war room, Blair said. From there it would go to the men and women who would turn the launch keys.

The Senate hearing was the first in Congress on presidential authority to use nuclear weapons since 1976, when a Democratic representative from New York, Richard L. Ottinger, pushed for the United States to declare it would never initiate a nuclear war. Forty-one years later, the US hasn’t ruled out first-strike nuclear options.

‘‘We are concerned that the president of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic, that he might order a nuclear weapons strike that is wildly out of step with US national security interests,’’ Senator Chris Murphy, Democrat of Connecticut, said at the outset of last week’s hearing.

The committee chairman, Senator Bob Corker, Republican of Tennessee, has also publicly questioned whether Trump’s aggressive rhetoric toward North Korea and other countries could lead the country into a world war. But Corker’s hearing produced little impetus for legislation to alter the presidential powers.

James Acton, codirector of the nuclear policy program at the Carnegie Endowment for International Peace, saw politics at play, but said the subject was “one we should be debating irrespective of who the president is.’’

Acton said a president rightly has unchecked authority to use nuclear weapons in response to an actual or imminent nuclear attack.

But he said a president considering a first strike should be required to consult in advance with the secretaries of state and defense, and the attorney general, and get approval from two of the three before acting.

Matthew Waxman, a professor at Columbia Law School, says changes of this sort would put a valuable check on the president and protect his nuclear authority from potential military insubordination.