A general view shows portraits of Russian President Vladimir Putin, left, and North Korea's leader Kim Jong-un with banners that translate as "Long live the undefeated friendship and unity of DPRK-Russia!" and "We warmly welcome Comrade Vladimir Vladimirovich Putin, the President of the Russian Federation," outside the Pyongyang Indoor Stadium in Pyongyang, June 20, displayed for a visit by Putin for a summit where he won a pledge of "full support" on Ukraine and signed a mutual defence pact. AFP-Yonhap

A State Department spokesperson reiterated "great concerns" Monday over growing military cooperation between North Korea and Russia, as the United States is set to host a summit of the North Atlantic Treaty Organization (NATO) in Washington this week.

Matthew Miller, the spokesperson, made the remarks as NATO allies are expected to highlight their solidarity against Russia's war operations in Ukraine during their summit set to take place in the U.S. capital from Tuesday through Thursday.

"I think we have made quite clear our great concern about increased collaboration between the DPRK and Russia ... the DPRK supporting Russia's war machine that is targeting, killing innocent civilians in Ukraine and violating the territorial integrity and sovereignty of another U.N. member in violation of multiple U.N. Security Council resolutions," he said during a press briefing.

DPRK stands for the North's official name, the Democratic People's Republic of Korea

"We will continue to work with our allies and partners to counter that relationship," he added.

The NATO summit comes after Russian President Vladimir Putin and North Korean leader Kim Jong-un signed a "comprehensive strategic partnership" treaty entailing a mutual defense clause during their summit in Pyongyang last month.

Burgeoning security cooperation between Moscow and Pyongyang has fueled concerns about its security implications on the Korean Peninsula and in Europe. (Yonhap)