Spokeperson for the State Department Matthew Miller speaks during a press briefing in Washington, D.C., U.S., June 7. Yonhap

The United States will oppose any cooperation between Russia and North Korea on Russia-occupied parts of Ukraine's territory, a State Department official said Wednesday, amid speculation that North Korean workers could potentially be sent there for reconstruction work.

Matthew Miller, the spokesperson, made the remarks as concerns have arisen that a recent security treaty between Russia and the North would likely bolster their military cooperation to the extent that Pyongyang could even commit its personnel to support Moscow in Russia-occupied areas in Ukraine, including those in Donetsk.

Last week, Russian President Vladimir Putin and North Korean leader Kim Jong-un signed a "comprehensive strategic partnership" treaty that commits each other to provide military assistance to the other "without delay" in the event of an armed invasion.

"Obviously, we have been quite concerned about the burgeoning relationship between North Korea and Russia. Obviously, Donetsk is part of Ukraine, not part of Russia," he told at a press briefing.

"So any kind of increased cooperation between those two countries, when it relates to activities on occupied Ukrainian territory, is something that we would oppose," he said.

On Tuesday, Pentagon Press Secretary Maj. Gen. Pat Ryder floated the possibility of North Korean troops becoming "cannon fodder" should they be sent to fight alongside Russian forces in Ukraine — an apparent display of U.S. opposition to the North's potential move to back Russia's war effort.

Commenting on the North's latest ballistic missile launch and its sending of trash-filled balloons to South Korea, Miller renewed calls for Pyongyang to refrain from "provocative" actions and return to diplomacy. (Yonhap)