Russian President Vladimir Putin said on Friday that it was Russia's business whether or not it decided to use North Korean troops, and that if Ukraine wanted to join Nato then Moscow could do what it wanted to ensure its own security.

The United States said on Wednesday that it had seen evidence that North Korea has sent 3,000 troops to Russia for possible deployment in Ukraine, a move that the West is casting as a significant escalation of the Ukraine war.

Ukraine's military intelligence service said that the first North Korean units trained in Russia had been deployed in the Kursk region, a Russian border area where Ukrainian forces took a chunk of Russian land in August.

"When we have to decide something, we will decide... but it is our sovereign decision whether we will apply it, whether we will not, whether we need it," Putin told Russian state television. "This is our business."

Putin said that the West repeatedly said that it was up to Ukraine how it ensured its security – "with or without Nato".

"The sooner they realise the futility of such an approach in relations with Russia, the better it will be for everyone, and perhaps, above all, for themselves," Putin said.

Meanwhile, Ukrainian President Volodymyr Zelensky on Friday said Russia could send North Korean troops into battle as early as Sunday and urged international pressure on the Kremlin and Pyongyang.

"According to intelligence reports, on October 27-28, Russia will use the first North Korean military in combat zones," the Ukrainian leader said on social media, calling for "tangible pressure" to be placed on Russia and North Korea.

Putin and North Korean leader Kim Jong Un signed a comprehensive strategic partnership agreement during Putin's visit to Pyongyang in June.

Article 4 of the agreement says: "If one of the parties is subjected to an armed attack by any state or several states and thus finds itself in a state of war, the other party will immediately provide military and other assistance with all means at its disposal."

Putin on Thursday did not deny US claims that North Korea had sent troops to Russia but said it is up to Moscow how to run its mutual defence clause with Pyongyang and accused the West of escalating the Ukraine war. (Reuters/AFP)