The US government is preparing "additional actions" in coming days against those responsible for the violent crackdown and military coup in Myanmar, US national security adviser Jake Sullivan said on Sunday.
"We will continue coordinating closely with allies and partners in the Indo-Pacific region and around the world to hold those responsible for violence to account," Sullivan said in a statement.
"We are preparing additional actions to impose further costs on those responsible for this latest outbreak of violence and the recent coup. We will have more to share in the coming days," he added.
Washington has already imposed a round of sanctions on Myanmar military leaders since the army seized power.
Myanmar security forces opened fire on protestors Sunday, killing at least eight in the bloodiest action so far to smother opposition to the military coup four weeks ago, AFP reported.
The junta is battling to contain a massive street movement demanding it yield power and release ousted civilian leader Aung San Suu Kyi, who was detained along with top political allies at the start of the month.
Sunday brought a significant escalation in force, with fatal shootings of protesters in at least four cities around the country, as police and soldiers attempted to bring the civil disobedience campaign to heel.
The United Nations condemned the deadly crackdown and said it had received credible information that as many as 18 people may have been killed.
"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 UN human rights office, said.
AFP could not independently confirm the UN toll.
Three men were killed and at least 20 others injured when security forces moved on a rally in the southern coastal hub of Dawei.