Ukraine's President Volodymyr Zelensky has denied his country carried out an alleged drone attack on the Kremlin, which Russia says was an attempt on President Vladimir Putin's life.

"We don't attack Putin or Moscow. We fight on our territory. We are defending our villages and cities," he said, speaking on a visit to Finland. 

The Russian president's office said defences downed two drones overnight.

It threatened to retaliate when and where it considered necessary.

Unverified footage circulating online shows smoke rising over the Kremlin - a large government complex in central Moscow - early on Wednesday. A second video shows a small explosion above the site's Senate building, while two men appear to clamber up the dome.

The Russian presidency said Ukraine had attempted a strike on Mr Putin's residence in the Kremlin and described it as "a planned terrorist act and an assassination attempt on the president".

Officials said two drones targeting the complex had been disabled using electronic radar assets, adding that President Putin had not been in the complex at the time of the alleged attack.

But Ukraine has said the Russian accusations are merely a pretext for massive attacks on its territory and the US says it is treating the Russian claims with a lot of caution.

Russia's Medvedev calls for 'elimination' of Zelenskyy after drone attack

Russian ex-president Dmitry Medvedev said on Wednesday in a post on social media that an alleged overnight drone attack on the Kremlin left Moscow with no options but to "eliminate" Ukrainian President Volodymyr Zelenskyy and his "clique" in Kyiv.