The Russian Foreign Ministry has expressed deep regret over North Korea’s second nuclear test.

In a statement released Thursday, the ministry said it conveyed its discontent to South Korea’s Ambassador to Moscow, Kim Young-jae. The statement also urged Pyongyang to swiftly return to the stalled six-way nuclear talks.

Russian officials said earlier that the North’s second nuclear test on Monday had raised tensions in the Northeast Asian region and had violated U.N. Resolution 1718.

Russia also postponed indefinitely a meeting with the North on commerce, economy and science technology. The move suggests Moscow could be taking an unprecedented hard-line stance toward the North’s test.