North Korea's efforts to develop its nuclear weapons program now make it the greatest threat to the U.S. and international security, Defense Secretary Jim Mattis said Monday.

In his prepared remarks before the House Armed Services Committee, the top defense official and former Marine Corps four-star general singled out Pyongyang in describing global dangers, saying North Korea is "the most urgent and dangerous threat to peace and security."

"North Korea's continued pursuit of nuclear weapons and the means to deliver them has increased in pace and scope," Mattis said, according to a copy of his remarks. "The regime's nuclear weapons program is a clear and present danger to all, and the regime's provocative actions, manifestly illegal under international law, have not abated despite United Nations censure and sanctions."

The severity of the threat posed by North Korea highlights the advances the regime has made in recent years, amid a recent spike in its testing of nuclear weapons and short-, medium- and long-range missiles. The hermit kingdom's leader, Kim Jong Un, appears to have staked his country's future, despite furthering its international isolation, on becoming a nuclear weapons power.

Mattis' latest assessment illuminates how quickly North Korea, in its pursuit of a nuclear weapon, has surpassed other international threats. When asked before Congress in the summer of 2015 about the greatest threats facing the U.S., most of the newly nominated members of the Joint Chiefs of Staff pointed to Russia, which unlike the Islamic State group posed an existential threat to the homeland.

"Russia presents the greatest threat to our national security," Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs who testified alongside Mattis on Monday, said at the time, calling Moscow's behavior "nothing short of alarming."

Since then, the Pentagon has refused to specify how close North Korea may be to developing a nuclear warhead, though experts believe Pyongyang is at most years away from perfecting the technology. Its scientists have conducted nuclear tests as recently as last year that resulted in a blast more powerful than the bomb the U.S. dropped on Hiroshima in 1945, though its ability to miniaturize and affix a nuclear device to an intercontinental ballistic missile remains unclear.

Pyongyang claims to have missiles capable of reaching the U.S. mainland, and its Taepodong-2 missile, considered operational, has a range that could reach the state of Maine.