US Secret Service agents shot and wounded a man who was apparently armed outside the White House on Monday, President Donald Trump said, after being briefly whisked away in the middle of a press conference.

A 51-year-old male approached a Secret Service officer standing at the corner of 17th Street and Pennsylvania Avenue, one block from the White House, according to Tom Sullivan, the chief of the Secret Service Uniformed Division.

The suspect told the officer he had a weapon and ran "aggressively" towards him, drawing an object out of his clothes, Sullivan said in a statement that was broadcast on Twitter. The secret service had to fire, he added.

Both the suspect and the officer were taken to hospital, he said, without giving more information.

As the president was speaking to reporters in the White House briefing room, a Secret Service bodyguard abruptly approached him and interrupted, saying in a quiet voice, "Sir, could you please come with me?"

Trump and staff members left. Doors to the briefing room, still filled with journalists, were locked.

Outside, black-clad Secret Service agents with automatic rifles could be seen rushing across the lawn and took up positions behind trees. According to Fox News, whose camera team was outside, two shots were heard.

Minutes later, Trump reappeared at the press conference and announced that someone had been shot by the Secret Service outside the White House grounds.

Trump, a Republican who faces a tough and bitterly divisive election on November 3, said he knew nothing about the identity or motives of the person shot. "It might not have had anything to do with me," he said.

Returning to the podium after the disruption, Trump appeared calm.

He resumed his virulent criticism of opposition Democrats and praise for his handling of the coronavirus crisis, which polls show two-thirds of Americans believe he has botched.

Questioned on whether the security incident had rattled him, Trump answered: "I don't know, do I seem rattled?"

"It's unfortunate that this is the world, but the world's always been a dangerous place," he said.

Trump went on to praise the US Secret Service, which is in charge of guarding the president, as "fantastic people, the best of the best."

"I feel very safe with Secret Service," he said. "A lot of terrific-looking people ready to go if something was necessary."