WASHINGTON, Feb. 28 (CNA) - 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 (Feb 28).

"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.

US Secretary of State Antony Blinken on Sunday also condemned what he called "abhorrent violence" by Myanmar security forces against the protesters.

"We stand firmly with the courageous people of Burma and encourage all countries to speak with one voice in support of their will," Blinken said on Twitter.

He said the US "will continue to promote accountability for those responsible".

At least 18 people were killed after Myanmar police fired on protesters around the country, the United Nations said, calling on the international community to act to stop the repression.

"Police and military forces have confronted peaceful demonstrations, using lethal force and less-than-lethal force that – according to credible information received by the UN Human Rights Office – has left at least 18 people dead and over 30 wounded," the UN human rights office said.

Myanmar has been in chaos since the army seized power and detained elected government leader Aung San Suu Kyi and much of her party leadership on Feb 1, alleging fraud in a November election her party won in a landslide.

The coup, which brought a halt to tentative steps towards democracy after nearly 50 years of military rule, has drawn hundreds of thousands onto the streets and the condemnation of Western countries.

Crowds of demonstrators came under fire in various parts of the biggest city of Yangon after stun grenades, tear gas and shots in the air failed to break up their protests.

Across the country, protesters wearing plastic work helmets and with makeshift shields faced off against police and soldiers in battle gear, including some from units notorious for tough crackdowns on ethnic rebel groups in Myanmar's border regions.