She made the remarks when Myanmar security forces killed at least 18 protesters and injured over 30 others around the country on Sunday in the bloodiest day since the February 1 coup, according to the U.N. Human Rights Office.
"We know Japan has developed ties with both Aung San Suu Kyi and the Tatmadaw", she said, referring to the ousted civilian leader and the armed forces. At least two dozen journalists have been detained since the military's takeover.
At least 18 people were killed Sunday as troops and police fired at demonstrators in cities across Myanmar, according to the United Nations, which cited its own credible information.
Teacher Tin New Yee died after police swooped to disperse a teachers' protest with stun grenades, sending the crowd fleeing, her daughter and a fellow teacher said.
Journalists have found themselves targeted by police and soldiers as they try to capture the unrest on the streets.
The AP called for his immediate release.
Ian Phillips, AP vice president for global news, said: "Independent journalists must be allowed to freely and safely report the news without fear of retribution. AP decries in the strongest terms the arbitrary detention of Thein Zaw", said Ian Phillips, the AP's vice president for worldwide news.
British Foreign Secretary Dominic Raab said Monday that Britain is also considering further actions against Myanmar.
Hundreds of thousands of people have taken to streets regularly over the past month to oppose the coup.
A committee representing lawmakers who won seats in the November election said at least 26 people were killed in the violence on Sunday, which Reuters was unable to verify.
AFP reported that in Yangon, demonstrators were seen using makeshift items like bamboo poles, sofas and even tree branches to erect barricades across streets.
"Now people are regrouping and protesting again", Yan said around 1 p.m. local time, saying the violence has made protesters "angrier" rather than scared.
Witnesses to Sunday's protests say police used tear gas, rubber bullets, water cannon and in some cases, live ammunition in Yangon, Myanmar's biggest city.
Some protesters called on Monday for destruction of surveillance cameras used by authorities, and shared pepper spray recipes on social media.
"We condemn the Burmese security forces' abhorrent violence against the people of Burma & will continue to promote accountability for those responsible", Secretary of State Antony Blinken tweeted Sunday, using the country's old name.
US officials including Secretary of State Antony Blinken also condemned the violence.
White House national security adviser Jake Sullivan issued a statement saying the United States is "alarmed" by the violence and stands in solidarity with Myanmar people "who continue to bravely voice their aspirations for democracy, rule of law, and respect for human rights".
Regarding whether Japan would impose sanctions on Myanmar over the military coup, including by withholding financial aid, Kato said the government is "closely watching the situation" and considering its options.
Suu Kyi was detained in Naypyidaw, the nation's capital, before dawn on the day of the coup, and had not been since in public since.
The Assistance Association for Political Prisoners said at least 270 people were detained on Sunday, from a total 1,132 it said had been picked up since the coup.
The calls for Japanese action came as Prime Minister Yoshihide Suga's government is considering halting new official development assistance projects in Myanmar in response to the coup and the rising death toll from the use of force, including shootings, by the Myanmar security apparatus against peaceful protests.
Crowds also gathered in other parts of the country, media reported.
State-run MRTV television said more than 470 people had been arrested on Saturday.
The teachers tried to assemble early, but police threw stun grenades and charged in to break up the protest. By subscribing, you can help us get the story right.
"We strongly condemn the escalating violence against protests in Myanmar and call on the military to immediately halt the use of force against peaceful protesters", Ravina Shamdasani, spokeswoman for the United Nations human rights office, said.