Ukrainian President Volodymyr Zelensky released a self-shot video from central Kyiv on Friday vowing alongside key aides to stay and defend the capital against the Russian invasion.

"We're all here. Our military is here. Citizens in society are here. We're all here defending our independence, our country, and it will stay this way," Zelensky said, standing outside the presidency building.

Wearing olive green military-style clothing and standing with his prime minister, chief of staff and other senior aides, Zelensky appeared to be responding to pressure from Russia's President Vladimir Putin.

Russian troops clashed briefly with Ukrainian forces for the first time within Kyiv itself on Friday.

Larger forces are bearing down on the capital and the city was bracing for a possible night of air strikes.

Meanwhile, in a televised address from Moscow, Putin branded Zelensky's government "terrorists" and "a gang of drug addicts and neo-Nazis", urging the Ukrainian military to mutiny.

Zelensky has received support from many Western leaders for his determination to resist Putin's invasion, and European Union chief Charles Michel embedded the Ukrainian leader's video in a tweet.

"The spirit of a free and democratic Ukraine is strong," Michel said.

The European Union is about to impose tough new economic penalties on Russia over the invasion, and Putin himself has been put on a sanctions list for the first time.

But Kyiv has been pushing EU and NATO members to do more, in particular through supplying advanced weaponry and excluding Russia from world financial networks.