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

"We really hold the military leadership accountable for what's happening," said Tillerson, adding that the US was "extraordinarily concerned" by the situation.

"Washington understands Myanmar has a militancy problem, but that the military must be disciplined and restrained," he said.

Tillerson did not say if the US would take action against the leaders over an offensive that has driven more than 500,000 of the minority group out of the country into neighboring Bangladesh.

Rohingya Muslims have fled Myanmar in large numbers since late August when Rohingya insurgent attacks led to a military response, with accusations of arson, killings and rape.

"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 [sic] to decide what direction do they want to play in the future of Burma?'"

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

Lawmakers urge US travel bans

Forty-three US lawmakers on Wednesday called on the Trump administration to bring back US travel bans on Myanmar's military leaders and to prepare sanctions against those responsible for the crackdown.

The request - made in a letter sent to Tillerson by Republican and Democratic members of the House of Representatives - said the 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.

The EU and the US have been considering targeted sanctions against Myanmar's military leadership, although they are wary of action that could hurt the wider economy or destabilize already tense ties between the country's effective leader Aung San Suu Kyi and the army.

UN laments response to atrocities

Two UN special advisers said on Wednesday that Myanmar's government had failed to meet its international obligationsto protect Rohingya Muslims from the atrocities in Rakhine state.

The UN special adviser on the prevention of genocide, Adama Dieng, and the special adviser on the responsibility to protect, Ivan Simonovic, said the international response to the crisis had been a failure.

"Despite warnings issued by us and by many other officials, the government of Myanmar has failed to meet its obligations under international law and primary responsibility to protect the Rohingya population from atrocity crimes," they said in a joint statement.

"The international community has equally failed its responsibilities in this regard," the advisers added.

The UN Security Council has called on Myanmar to end military operations in Rakhine state, grant access to aid workers and allow the safe return of the refugees.

A recent report by the UN human rights office accused Myanmar of seeking to expel the Rohingya permanently, by planting land mines at the border with Bangladesh.

The UN has requested $430 million (€390 million) to enhance relief efforts for the refugees, with UN rights experts expressing concern that human rights violations may amount to "crimes against humanity."

Images of burning villages

New York-based Human Rights Watch (HRW) on Wednesday accused Myanmar soldiers of carrying out mass killings, torture and arson attacks on Rohingya Muslims in Rakhine. 

The watchdog said in a statement it had also heard allegations of sexual assaults on Rohingya women by Myanmar soldiers in Maung Nu, a Muslim village. 

HRW said that Myanmar soldiers had beaten, sexually assaulted, stabbed and shot villagers who had gathered for safety in a residential compound two days after Rohingya militants attacked a local security outpost and military base on August 25.

Human rights groups using satellite images said recently that about half of more than 400 Rohingya villages in the north of Rakhine state have been razed in the violence.

jbh/jm (Reuters, AFP, dpa)