Civilian death toll in Ukraine at 227: UN

"We're looking very closely at what's happening in Ukraine right now including what's happening to civilians," Blinken told reporters.

"We're taking account of it, we're documenting it and we want to ensure, among other things, that there's accountability for it." 

The UN human rights office said in a statement most of the casualties - which were tallied as of midnight on March 1 local time - were caused by the use of explosive weapons with a wide impact area, "including shelling from heavy artillery and multi-launch rocket systems, and air strikes".

It said it believes the true toll in the week-long conflict is "considerably higher", especially in government-controlled territory, due to reporting delays in some areas where fighting has been most intense.

The invasion has yet to achieve Russian President Vladimir Putin's aim of overthrowing Ukraine's government but it has sent more than 870,000 people fleeing to neighbouring countries and jolted the global economy as governments and companies line up to isolate Moscow.