His comments came after Ukrainian President Volodymyr Zelensky said fresh Russian strikes on a market and filling station had killed six people and wounded several more in the frontline region of Donetsk.

With the one-year mark of the conflict approaching, Russian forces have missed most of their key military goals including toppling the Ukrainian government, capturing the Donbas region and annexing four territories.

Putin's own menacing language, and the battlefield stalemate, have raised fears Russia could resort to its nuclear arsenal to achieve a military breakthrough.

On Wednesday, Putin warned that the threat of nuclear war was growing.

"Such a threat is rising. Why make a secret out of it here?" he said at a meeting of his human rights council.

He added however that Russia would use a nuclear weapon only in response to an enemy strike.

"When we are struck, we strike back," he said, stressing that Moscow's strategy was based on "so-called retaliatory strike" policy.

Moscow had expected fighting to last just days before Ukraine's capitulation, but on Wednesday Putin warned results could be a long time coming.

"As for the long process of (seeing) results of the special military operation, of course, this is a lengthy process," Putin said.

But he praised the announced annexation of four Ukrainian territories into Russia after Moscow proxies held referendums -- denounced in the West as a sham -- and announced their integration in September.

"New territories appeared -- well, this is still a significant result for Russia and this is a serious issue," Putin said.

The Russian leader formalised the annexation of the four southern and eastern territories -- Donetsk, Lugansk, Kherson and Zaporizhzhia -- at a ceremony in the Kremlin in September.

- Russian shelling on market -

But Russian troops at no point entirely controlled any of the territories and last month were forced out from the capital of the southern Kherson region after a months-long Ukraine counter-offensive.

In September Putin announced Russia was mobilising hundreds of thousands of people to bolster Moscow's struggling forces after a series of battlefield setbacks, particularly in the Kharkiv region in northeast Ukraine.

On Wednesday he said half the Russians called up for military service in September had been deployed to Ukraine.

"Out of 300,000 of our mobilised fighters, our men, defenders of the fatherland, 150,000 are in the area of operations," said Putin, adding that about 77,000 were in combat units.

Since the capture of Kherson city, fighting in Ukraine has focused on the industrial Donbas region, where Russian forces have been pushing to capture the frontline city of Bakhmut.

Zelensky -- after visiting the region this week -- said Wednesday that Russian forces had killed six civilians and injured several in a recent bout of shelling.

"Terrorists attacked the peaceful city of Kurakhove," he said in a statement on social media.

"A market, a bus station, gas stations, and residential buildings came under fire. At least six civilians were killed, five were wounded."

The Donetsk region has been partially controlled by Russian forces since 2014, when Moscow-backed separatists wrested control of the Donbas near the Russian border and Moscow annexed the Crimean peninsula.