The United States holds Myanmar's military leadership responsible for its harsh crackdown on the Rohingya Muslim minority, the US secretary of state said.

Rex Tillerson, however, stopped short of saying on Wednesday whether the US would take any action against Myanmar's military leaders over an offensive that has driven more than 500,000 Rohingya out of the country.

"The world can't just stand idly by and be witness to the atrocities that are being reported in the area," Tillerson told Washington's Center for Strategic and International Studies think-tank.

Forty-three US legislators urged the Trump administration to reimpose US travel bans on Myanmar's military leaders and prepare targeted sanctions against those responsible for the crackdown.

The request, in a letter to Tillerson from members of the House of Representatives, said Myanmar authorities "appear to be in denial of what has happened", and called for Washington to take "meaningful steps" against those who have committed human rights abuses.

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Rohingya have fled Myanmar in large numbers since late August when Rohingya rebel attacks sparked a ferocious military response, with the fleeing people accusing security forces of arson, killings and rape.

Tillerson said Washington understood Myanmar had a problem with armed groups, but the military had to be disciplined and restrained in the way it dealt with this and to allow access to the region "so that we can get a full accounting of the circumstances".

"Someone, if these reports are true, is going to be held to account for that," Tillerson said. "And it's up to the military leadership of Burma to decide what direction they want to play in the future of Burma."

Tillerson said Washington saw Myanmar, which is also known as Burma, as "an important emerging democracy," but the Rohingya crisis was a test for the power-sharing government.