Putin on Wednesday ordered a partial mobilisation of 300,000 reservists for his country's military campaign in Ukraine, news outlets reported.
According to the Reuters news agency, the move marks Russia's first such mobilisation since World War II and threatens to cause a major escalation of the war in Ukraine.
"In this war, there is only one aggressor, Russia, and one aggressed country, Ukraine," the EU's Michel said in a tweet on Wednesday.
He pledged that the "EU's support to Ukraine will remain steadfast."
Michel, who is president of the council representing the bloc's 27 member states, was tweeting after Putin backed a plan to annex four Ukrainian provinces and warned the West that he was ready to use nuclear weapons to defend Russia, Reuters reported.
Putin 'interested in escalating war of aggression': spokesman
Meanwhile, Peter Stano, a foreign policy spokesman for the European Commission, the EU's executive, was quoted as saying that Putin's announcement of a partial mobilisation showed his desperation.
Stano told a news briefing that Putin's move was further proof that "he is not interested in peace, that he's interested in escalating this war of aggression," Reuters reported.
"This is also yet another sign of his desperation with how his aggression is going against Ukraine ... he is only interested in further advancing and continuing his destructive war, which has had already so many bad consequences worldwide," Stano said.
He was speaking as international leaders were meeting at the United Nations General Assembly in New York to discuss a joint response to Putin's latest steps in the war against Ukraine, according to news reports.
"Putin is doing a nuclear gamble," Stano told reporters, as quoted by Reuters. "He's using the nuclear element as part of his arsenal of terror. It's unacceptable."