Russian President Vladimir Putin on Wednesday signed legislation to annex four territories of Ukraine, government documents showed.

The Donetsk, Lugansk, Kherson and Zaporizhzhia regions are "accepted into the Russian Federation in accordance with the constitution of the Russian Federation", the documents said.

Putin also signed decrees formally appointing the current Moscow-backed heads of the four regions as their acting leaders.

Earlier, the annexation legislation was unanimously supported by both Russia's lower and upper chambers of parliament.

Putin last Friday presided over a grand ceremony at the Kremlin during which he signed agreements with the Moscow-installed leaders of the four regions to become subjects of the Russian Federation, despite condemnation from Kyiv and its allies in the West.

The four territories create a crucial land corridor between Russia and the Crimean Peninsula, which was annexed by Moscow in 2014.

Together, the five regions make up around 20 percent of Ukraine.

But Russian forces do not have complete control over Kherson or Zaporizhzhia and the Kremlin has not yet confirmed what areas of those regions are being annexed.