WASHINGTON • US Secretary of State Rex Tillerson yesterday said he has never considered resigning from his post, amid tensions with President Donald Trump whom he had reportedly branded a "moron" during a Pentagon meeting.

"My commitment to the success of our President and our country is as strong as it was on the day I accepted his offer," said Mr Tillerson, the country's top diplomat, during a hastily called press conference at the State Department.

"I am here for as long as the President feels I can be useful to achieving his objectives."

Mr Tillerson spoke after NBC News, citing "multiple senior administration officials", reported that a few days after a heated meeting on July 20, Vice-President Mike Pence met Mr Tillerson to ask him to show respect.

When asked by a reporter to comment on his alleged "moron" remark, Mr Tillerson did not deny outright the name calling. "I'm not gonna deal with petty stuff like that," he said.

The NBC report, which includes a formal denial of the reported outburst from a senior Tillerson aide, came just days after Mr Trump upbraided Tillerson for "wasting his time" trying to negotiate with North Korea.

Minutes after Mr Tillerson wrapped up his media conference, President Trump, who was heading to Las Vegas to visit a city shaken by the deadliest shooting spree in modern US history, sent a tweet.

"The @NBCNews story has just been totally refuted by Sec. Tillerson and @VP Pence. It is #FakeNews. They should issue an apology to AMERICA!" he wrote.

State Department and White House staff did not immediately respond to media requests for comment on the NBC report, but Mr Trump had fired off a tweet earlier in the day which may have been provoked by the report.

"Wow, so many Fake News stories today. No matter what I do or say, they will not write or speak truth. The Fake News Media is out of control!" he wrote, without elaborating.