Members of Congress are sharpening their criticism of Myanmar's crackdown that has forced a half-million Rohingya Muslims to flee to Bangladesh. Republicans and Democrats said Thursday they want the Trump administration to consider sanctions against the perpetrators and to re-evaluate U.S. policy toward the Southeast Asian nation.

Twenty-one senators said in a letter to Secretary of State Rex Tillerson that there's a risk of genocide against the Rohingya, who have fled en masse in the past month after Myanmar security forces responded to insurgent attacks with "clearance operations" that have left hundreds dead and thousands of homes burned.

The senators said that response has been "extraordinarily disproportionate."

A copy of the letter was obtained by The Associated Press. It urges the Trump administration to hold perpetrators of atrocities in Myanmar's Rakhine State accountable under international law and U.S. law that allows the president to impose sanctions on individuals responsible for extrajudicial killings, torture and other gross violations of human rights in any foreign country.

"Unless immediately addressed, this crisis will have profound long-term consequences for Burma, the region, and the world," the letter says, using the alternative name for Myanmar. Long-standing sectarian tensions between the Rohingya and majority Buddhists have bubbled to the surface as the country has opened up after decades of oppressive military rule.

Signatories of the letter include Ben Cardin, top-ranking Democrat on the Senate Foreign Relations Committee, and John McCain, Republican chairman of the Senate Armed Services Committee, who recently nixed plans to expand military ties with Myanmar.

The senators' call for action came as the U.N. Security Council held its first public session Thursday on the refugee crisis. That will also crank up diplomatic pressure on Myanmar, which has denied accusations of ethnic cleaning and sexual violence against the Rohingya. Human rights groups say more than 200 Rohingya villages have been burned in the crackdown.

U.S. Ambassador Nikki Haley called it "a brutal, sustained campaign to cleanse the country of an ethnic minority." She said the council should consider action against Myanmar security forces "who are implicated in abuses and stoking hatred among their fellow citizens." She urged countries to suspend any provision of weapons to Myanmar.

The senators are also calling for Myanmar to allow access to international humanitarian groups and for the U.S. to provide more aid as Bangladesh struggles to cope with the massive and sudden influx of stricken people. The administration has already announced $32 million in assistance, bringing U.S. funds committed for the crisis to $95 million.

The administration was initially muted about the crisis but has become more outspoken as the gravity of the situation has become apparent. At the U.N. last week, Vice President Mike Pence accused Myanmar's security forces of "terrible savagery."

"We need to put as much pressure as possible on the Burmese government. They're responsible for what's happening, and they know that," Cardin told reporters Wednesday. He said the U.S. needs to send a clear message that it could revert to its former policy of sanctions against Myanmar. Those restrictions were lifted as it shifted toward democracy.

At a House foreign affairs hearing Wednesday, lawmakers strongly criticized not just the conduct of Myanmar's military but its civilian leader Aung San Suu Kyi, a former political prisoner who took power last year after an election victory. She said last week the Rohingya refugees would be allowed to return from Bangladesh if they passed a "verification" process.

Democratic Rep. Brad Sherman said that amounted to a Catch-22. He said discriminatory Myanmar law makes it impossible for Rohingya to prove their nationality. He said it was "outrageous" that Rohingya who had lived in Myanmar for generations are denied citizenship. He also said the U.S. needed to re-evaluate its policy of having lifted sanctions.

Republican Rep. Ted Yoho called for $63 million in planned U.S. aid to Myanmar for fiscal 2018 to be suspended because of the crackdown in Rakhine.

"We're in the 21st century and I see these atrocities going on that makes (you) not even want to be part of the human race," he said.