South Korea's vice foreign minister, Kim Hong-kyun, in the phone call with U.S. Deputy Secretary of State Kurt Campbell, said that Putin's visit should not result in deeper military cooperation between Pyongyang and Moscow in violation of U.N. Security Council resolutions, the ministry said.

Echoing Kim's concerns, Campbell pledged continued cooperation to tackle potential regional instability and challenges caused by the trip.

"While closely monitoring related developments, the two sides agreed to resolutely respond through airtight cooperation to North Korea's provocations against South Korea and actions that escalate tensions in the region," the ministry said in a statement.

On Wednesday, a senior official at Seoul's presidential office said Putin was expected to visit North Korea "in the coming days". Russia's Vedomosti newspaper on Monday reported Putin would visit North Korea and Vietnam in the coming weeks.

Civilian aircraft have been cleared from Pyongyang's airport and there are signs of preparations for a possible parade in the capital's Kim Il Sung Square, NK PRO, a Seoul-based website, reported this week, citing commercial satellite imagery.

When Sergei Shoigu, then Russia's defence minister, visited Pyongyang last year to jumpstart the two countries' warming ties, he accompanied Kim to a parade and saluted as North Korea's banned nuclear-tipped missiles rolled by.

Speaking at the Stimson Center think tank in Washington on Wednesday, Campbell said the United States has a very good understanding of what North Korea has provided Russia, which he said has had "a substantial impact on the battlefield".

Less clear, he said, is what Russia has provided North Korea.

"Hard currency? Is it energy? Is it capabilities that allow them to advance their nuclear or missile products? We don't know. But we're concerned by that and watching carefully," he said.