Belgium Foreign Minister Sophie Wilmes. AP
"Russia has made the deliberate choice to violate international law ... and to undermine the sovereignty and territorial integrity of Ukraine," the minister said in a statement.
Ukraine mobilized reservists and Moscow evacuated its Kyiv embassy on Wednesday, with 150,000 Russian troops massed on its borders, according to Western capitals.
Wilmes saw ambassador Alexander Tokovinin during the afternoon, and "recalled Belgium's strong condemnation", urging Russia "to reconsider its decision".
President Vladimir Putin underlined Tuesday that Russia recognized the separatists' independence and sovereignty over all the regions of Lugansk and Donetsk, and not just areas they control.
The EU announced new sanctions against Russia on Tuesday with official details due to be revealed shortly.
"They are the first round of sanctions which can be strengthened as the situation changes," Wilmes said.
She stressed that the "path of diplomacy remains open".
"Belgium is waiting now for Russia's acts rather than intentions to show a sincere commitment for de-escalation and for the search for peaceful solutions to current tensions," the minister added.