The United States is "incredibly concerned" about long-suspected military cooperation between North Korea and Iran, a State Department spokesperson said Tuesday.

Matthew Miller, the spokesperson, made the remarks in response to a question about whether America has concerns about Iran working with Pyongyang on nuclear and ballistic missile programs.

"Certainly, that's something we've been incredibly concerned about," Miller told a press briefing.

Having established diplomatic ties in 1973, Pyongyang and Tehran are known to have close ties while under biting international sanctions for their weapons programs.

In a separate briefing, Pentagon Press Secretary Maj. Gen. Pat Ryder also responded to a question over cooperation between Iran and North Korea, and the possibility that North Korean weapons were used to attack Israel.

"I can't speculate. Certainly when it comes to threats posed by the DPRK and Iran, (it's) something that we take very seriously," he said, referring to the North by its official name, the Democratic People's Republic of Korea.

"When it comes to the Indo-Pacific region, just to make clear -- similar to our ironclad defense of Israel -- our relationship and our alliances with the Republic of Korea and Japan are also ironclad, and we will stand beside them to work together towards security and stability throughout the region," he added. (Yonhap)