The pair, who according to US media told officers they had driven from the Midwest state of Indiana, were charged with multiple weapons-related violations.

The Secret Service told AFP its officers "observed two individuals acting suspiciously near a vehicle," approximately three blocks from the embassy just after 9:00 am (1400 GMT).

The Secret Service was unable to give their names and provided no further details.

However, The Washington Post reported that officers seized a long gun and two handguns, and that one of the men was wearing military fatigues.

Ukrainian President Volodymyr Zelensky has urged any foreigners to come to Ukraine "and fight side by side with the Ukrainians against Russian war criminals," the country's general staff said.

A Ukrainian statement additionally called on volunteers to visit Ukrainian embassies in their respective countries.

Russia invaded Ukraine eight days ago and has faced fierce resistance by Ukrainians, in addition to deepening international isolation.