SEOUL — President Vladimir Putin of Russia and North Korea’s leader, Kim Jong Un, revived a Cold War-era mutual defense pledge between their nations on Wednesday, as the Kremlin deepened its security relationship with North Korea and vowed solidarity in challenging the United States. Neither Russia nor North Korea immediately released the text of the new treaty. But Putin, speaking at a joint briefing in Pyongyang after the two leaders signed the document, said the pact called for the nations to aid each other in the event of “aggression” against either country.

Kim claimed the new “treaty” elevated the two countries’ relations to an “alliance.” Putin did not say whether the new agreement would require immediate and full-fledged military intervention in the event of an attack, as the now-defunct 1961 treaty between Moscow and Pyongyang specified during the days of the Cold War. Advertisement The pledge of mutual assistance is likely to further alarm Washington and its allies.

It could presage not only deeper support by North Korea for Russia’s war in Ukraine but also greater support from Moscow in aiding Kim’s quest for better-functioning nuclear weapons, missiles, submarines, and satellites — a development that would increase anxiety among America’s Asian allies, especially South Korea. Both leaders heralded the agreement as the beginning of a new era in their relations. Putin, speaking on his first trip to North Korea in nearly a quarter-century, c.