Former U.S. Forces Korea Commander Curtis Scaparrotti has expressed serious concern about North Korea's missile capabilities after the communist nation carried out the latest test of its submarine-launched ballistic missile.

"Kim Jong-un and his regime continues to test and work on their ballistic missile capability, and with every launch, they're getting better and they're working out their problems," Scaparrotti, now NATO's supreme allied commander in Europe, told reporters at the NATO Summit in Warsaw on Saturday.

"And they are doing it with a missile now, the Musudan variant, that has regional range and the very same things that he learns is going to be transferred to his intercontinental ballistic missile capability, so it's a serious threat," he said, according to video footage of his remarks.

Asked if he has concerns about the North's capability to strike the mainland U.S., Scaparrotti said, "I do."

"As a commander, I have to be concerned about that. I've got to assume he's got the capability to do it," he said.

North Korea conducted the latest SLBM test Saturday, a day after the South and the U.S. announced a decision to deploy the Terminal High Altitude Area Defense missile defense system to better defend against the North's nuclear and missile threats.

The latest test, however, ended in failure, with the missile, known as the KN-11, exploding at an altitude of some 10 kilometers after being launched from a submerged 2,000-ton Sinpo-class submarine, according to South Korea's Joint Chiefs of Staff.

Scaparrotti led USFK from August 2013 until April this year.

He is the person who brought up the need for a THAAD unit in South Korea for the first time in June 2014.

The U.S. desire to put a THAAD battery in the South put Seoul in a dilemma as China has expressed strong opposition, saying the system can be used against it. Washington has repeatedly stressed that the system is purely defensive and aimed only at deterring North Korean threats.

North Korea's nuclear and missile tests in January and February ended the dilemma. Immediately after the North's Feb. 7 long-range rocket launch, South Korea and the U.S. decided to launch official talks about the THAAD deployment, and on Friday, the two sides officially decided to deploy the system. (Yonhap)