A top U.S. diplomat handling North Korea said Thursday that Russia remained committed to denuclearizing the reclusive state, despite a possible visit by the North's leader Kim Jong-un to Russia in May.

Ambassador Sung Kim, the U.S. special representative for North Korea policy, made the remarks upon his arrival at the Beijing airport for talks with his Chinese counterpart, Wu Dawei, after visiting Tokyo.

North Korea has confirmed the attendance of its leader for a May ceremony in Russia marking the Soviet victory over Nazi Germany in World War II, the Kremlin said in a statement on Wednesday, without mentioning Kim Jong-un's name.

If realized, it would be Kim's first foreign trip since taking the helm of North Korea in late 2011. Some analysts are concerned that closer ties between North Korea and Russia may complicate diplomatic efforts to denuclearize North Korea.

Asked about Kim Jong-un's possible visit to Russia in May, Sung Kim replied, "I have seen reports that the visit might be possible."

"The main thing is that Russians remain very much committed to denuclearization and the joint statement of the six-party process," he said.

"They have made it very clear that they would strongly oppose nuclear tests by North Korea and, in fact, they oppose continuing nuclear activities by North Korea."

The U.S. envoy further said he held "very productive" talks in Tokyo with his South Korean and Japanese counterparts and expects to hold "very substantive and detailed" talks with his Chinese counterpart in Beijing.

North Korea is seeking to deepen both diplomatic and economic ties with Russia at a time when its political relationship with China remains frosty amid international pressure over its nuclear ambitions and dismal human rights record.

Russian President Vladimir Putin, who has been under intense pressure over the conflict in Ukraine, is also eager to bolster ties with North Korea in an apparent effort against America's pivot to Asia. Putin invited Kim Jong-un to attend the May 9 ceremony during a meeting with Kim's special envoy in November.

China is North Korea's main economic benefactor, but political ties between the allies remain strained, particularly after the North's third nuclear test in early 2013. (Yonhap)