Russia's President Vladimir Putin and North Korea's leader Kim Jong Un react during a walk in the garden of the Kumsusan Guesthouse in Pyongyang, North Korea

North Korea and Russia have agreed to provide immediate military assistance if either faces armed aggression, according to the full text of a landmark pact released on Thursday by Pyongyang after a visit by Russian President Vladimir Putin.

South Korea replied by calling an emergency meeting of its national security council and announcing that it might reconsider delivering guns to Ukraine, something it had previously rejected.North Korea's state media released the "Treaty on Comprehensive Strategic Partnership" hours after Putin left for Vietnam, effectively renewing a defunct mutual defence accord from the 1960s.

The deal, agreed on Wednesday by Putin and North Korean leader Kim Jong Un, includes cooperation in nuclear energy, space exploration, food and energy security, and is one of Moscow's most visible advances in Asia in recent years. Putin visited China last month, shortly after being inaugurated for a fifth term as president.

"In case any one of the two sides is put in a state of war by an armed invasion from an individual state or several states, the other side shall provide military and other assistance with all means in its possession without delay in accordance with Article 51 of the UN Charter and the laws of the DPRK and the Russian Federation," 

Article 51 of the U.N. Charter provides for the right of a member country to take individual or collective self-defence actions.

Kim echoed Putin's statement explicitly linking their deepening ties to fighting the "hegemonic and imperialist" policies of the West and the United State in particular, including its support for Ukraine.

Washington and Seoul have been increasingly alarmed by deepening military cooperation between Russia and the North, and have accused them of violating international laws by trading in arms for Russia to use against Ukraine. Ukrainian officials have said they have found North Korean missile debris inside their country. Russia and North Korea deny any arms trade.

Following a meeting of its national security council, South Korea said that it will boost security cooperation with the United States and Japan. Seoul will add 243 more products to a list of commodities barred from shipment to Russia, raising the total to 1,402, and will reconsider its position on arming Ukraine, according to national security advisor Chang Ho-jin.

Artyom Lukin, of Russia's Far Eastern Federal University, said the pact with North Korea would be "Moscow’s first defense alliance outside the post-Soviet space".

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"It remains to be seen how far and how deep Russia and North Korea will go in their alliance relationship this time," Lukin said. "Will North Korean troops appear in Ukraine? Will Russia provide military assistance to the North in possible North-South clashes over the disputed border in the Yellow Sea? Nothing is off the table now."

Cho Han-bum, of the Korea Institute for National Unification in Seoul, said the agreement was a big win for Russia, laying a legal framework for North Korea's support in the Ukraine war.

The wording leaves room for Moscow to avoid helping North Korea in border clashes or other future skirmishes if it chooses, he said.

PUTIN THANKS KIM FOR SUPPORT

On his first visit to Pyongyang since 2000, Putin thanked Kim for the support for Russian policy. Kim reaffirmed "unconditional" and unwavering support for "all of Russia's policies" including Putin's war with Ukraine.

Russia and North Korea face increasing international isolation, and the U.S. and its Asian allies are increasingly concerned about how much Russia will support North Korea, the only country to have tested a nuclear weapon this century.

The agreement said neither side would sign any treaty with a third country that infringes on the interests of the other and would not let its territory be used by any country to harm the other's security and sovereignty, KCNA said.

The two countries will take joint actions aimed at "strengthening defence capabilities to prevent war and ensure regional and international peace and security", it said.

South Korea regretted the agreement included a pledge of "military technology cooperation" which it said would violate U.N. Security Council resolutions on North Korea's weapons programs.

Russia, which has veto power at the Security Council, supported sanctions against North Korea enacted after Pyongyang tested a nuclear weapon in 2006, but has lately said sanctions should be changed. It vetoed the annual extension of inspectors enforcing the sanctions this year.

The White House did not immediately comment on the agreement's reported content.

Japan expressed "grave concerns" about Putin's vow not to rule out cooperation with Pyongyang on military technology.

Putin taunts West as North Korean trip confirmed

The reaction from China, the North's main political and economic benefactor, has been muted. A spokesperson for China's foreign ministry declined comment, calling it a bilateral matter between Russia and North Korea.

Ukrainian presidential aide Mykhailo Podolyak said Russia, a U.N. Security Council permanent member, had allowed "the most brazen nullification" of all sanctions imposed on North Korea to stop its weapons development.