The violent crackdown on demonstrators by Myanmar’s security forces showed that the junta will “sacrifice the lives of the people to serve the few,” Blinken said in a tweet.
“The courageous people of Burma reject the military’s reign of terror,” he added, using the name the country was known as until it was changed by its then-military rulers in 1989.
His condemnation came after dozens of people were killed during military crackdown Saturday in the deadliest bloodletting since the military took control of the country on Feb. 1, ousting the elected government of Aung San Suu Kyi.
"It's terrible. It's absolutely outrageous," said President Joe Biden on Sunday on his way back from Delaware to the White House. "And based on reporting I've gotten, an awful lot of people have been killed totally unnecessarily."
The online news site Myanmar Now reported late Saturday that the death toll had reached at least 114, including a 13-year-old girl. Independent monitoring group, Assistance Association for Political Prisoners, put Saturday's death toll at 90, adding that 423 people have been killed since the coup began.
NBC News could not independently verify these numbers.
Despite the violence, protesters returned to the streets Sunday to press their demands for a return to democracy in Yangon and Mandalay, the country's two biggest cities, and across the country. Some of the demonstrations were again met with police force as funerals for those slain on Saturday were held across the country.
In a strongly-worded statement Saturday, U.S. Ambassador to Myanmar, Thomas Vajda, also blamed the country’s security forces for “murdering unarmed civilians, including children, the very people they swore to protect.”
“These are not the actions of a professional military or police force,” Vajda said. “Myanmar’s people have spoken clearly: they do not want to live under military rule.”
The U.S. Embassy said in a tweet that shots were fired Saturday at its cultural center in Yangon, though no one was injured.
A joint statement from the defense chiefs of 12 countries, including the U.S., Germany, the U.K. and Japan among others, also condemned the violence. “We urge the Myanmar Armed Forces to cease violence and work to restore respect and credibility with the people of Myanmar that it has lost through its actions," their statement said.
In a tweet, Defense Secretary Lloyd Austin called reports of the violence “deeply disturbing.”
However, United Nations Special Rapporteur Tom Andrews called for a "robust, coordinated action" and not just words of condemnation to stop the bloodshed, saying in a statement they "ring hollow to the people of Myanmar while the military junta commits mass murder against them."
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The crackdown took place as Myanmar’s military celebrated the annual Armed Forces Day holiday with a parade in the country’s capital.
Junta chief Senior General Min Aung Hlaing did not directly refer to the protest movement when he gave his nationally televised Armed Forces Day speech before thousands of soldiers in Naypyitaw on Saturday. He referred only to “terrorism which can be harmful to state tranquility and social security,” and called it unacceptable.
In recent days the junta has portrayed the demonstrators as the ones perpetrating violence for their sporadic use of Molotov cocktails.
On Saturday, some protesters in Yangon were seen carrying bows and arrows.
In contrast, security forces have allegedly used live ammunition for weeks against what have still been overwhelmingly unarmed and peaceful crowds.
The Associated Press contributed to this report.