Japan is "seriously concerned" that Russian President Vladimir Putin did not rule out military and technical cooperation under a pact signed with North Korea on a visit to the isolated state, a Japanese government spokesman said Thursday.

Putin on Wednesday inked the strategic treaty, at a summit with North Korean leader Kim Jong Un in Pyongyang, that included a pledge to come to each other's aid if attacked.

Russia "does not rule out military-technical cooperation with the DPRK in connection with the treaty that was signed today," Putin said, using the acronym for North Korea's official name.

But that sort of cooperation "could be a direct violation of the relevant U.N. Security Council resolutions, in terms of its impact on the security environment surrounding our country and the region," Japan's top government spokesman Yoshimasa Hayashi told reporters at a regular briefing.

Japan is "seriously concerned about the fact that President Putin did not rule out military-technical cooperation with North Korea," he said.

Putin also said that U.N. sanctions on the North Korean regime should be "reviewed" — a call Hayashi on Thursday deemed "unacceptable."

"Despite the will of the international community, Russia has procured arms and ammunition, including ballistic missiles, from North Korea and used them in Ukraine in clear violation of the relevant U.N. Security Council resolutions," he said.

"The announcement (from Putin about reviewing U.N. sanctions on North Korea) is unacceptable."