Russia's President Vladimir Putin, left, and North Korea's leader Kim Jong- un attend a state reception in Pyongyang, June 19. Reuters-Yonhap

Russian President Vladimir Putin has signed off on a landmark defense pact with North Korea, a deal that comes amid reports that Pyongyang has despatched thousands of troops to fight against Ukraine.

Putin struck the deal during a visit to North Korea in June, but it had to pass through Russia's parliament and signed by the Kremlin leader to come into force.

The Kremlin published the signed law ratifying the treaty on its website on Saturday evening.

The agreement formalizes months of deepening security cooperation between the two nations, which were Communist allies throughout the Cold War.

North Korea has become one of the most vocal and important backers of Russia's full-scale offensive on Ukraine.

The deal obligates both states to provide military assistance "without delay" in the case of an attack on the other.

It also commits them to cooperate internationally to oppose Western sanctions and coordinate positions at the United Nations.

Putin hailed it in June as a "breakthrough document."

Citing intelligence reports, South Korea, Ukraine and the West say North Korea has deployed around 10,000 troops to Russia to fight against Ukraine.

When asked publicly about the deployment last month, Putin did not deny it, instead deflecting the question to criticize the West's support of Ukraine. (AFP)