South Korean Deputy Foreign Minister Kim Hong Kyung said during a phone call with U.S. Deputy Secretary of State Kurt Campbell that Putin's visit should not lead to deeper military cooperation between Pyongyang and Moscow in violation of U.N. Security Council resolutions.

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

"Closely following relevant developments, both sides agreed to resolutely respond through close cooperation to North Korea's provocations against South Korea and actions that increase tensions in the region", the ministry said in a statement.

A senior official in Seoul's presidential office said Wednesday that Putin is expected to visit North Korea "in the coming days." Russia's Vedomosti newspaper reported that Putin will visit North Korea and Vietnam in the coming weeks.

Speaking at the Stimson Center think tank in Washington, Campbell said the United States is very aware of what North Korea has provided to Russia, which he said has had a "significant impact on the battlefield".

What is less clear, he said, is what Russia has provided to North Korea.

"Is it hard currency? Is it energy? Is it capabilities that allow them to develop their nuclear or missile products? We don't know. But we are concerned about it and we are watching it closely", he said.

• It was earlier reported that Russian dictator Vladimir Putin plans to make a working visit to North Korea in the near future and later visit Vietnam.