The US remains "concerned" over North Korea's provocative actions and is keeping a close eye on Pyongyang amid fresh anxiety over its continued pursuit of nuclear and missile programs, the White House said Wednesday.

White House Press Secretary Josh Earnest made the remark during a regular press briefing after the North test-fired a submarine-launched ballistic missile in the East Sea. It flew over

500 kilometers, making it the longest flight by such a missile for the North, before landing within Japan's air defense identification zone.

"I think it should be a pretty good indication to you and everybody else that this is something that we're watching closely.

And we do continue to be concerned by the provocative destabilizing actions of the North Korean regime," Earnest said.

He noted that the US has worked "effectively" with Beijing to apply more pressure on the North and it is "critical" to form a united front against the North's nuclear and missile programs.

"You've heard me say on a number of occasions now that our ability to unite the international community -- not just our allies, but even some of our partners in this effort, like China -- to present a united front to North Korea and deepen their isolation will be critical to our ability to apply sufficient pressure to change their behavior," he said.

"That hasn't happened yet. And we're going to continue to work cooperatively in a coordinated fashion with the Chinese to try to steadily ratchet up that pressure," he added.

Asked whether the US could seek additional sanctions against the North for its latest missile launch, Earnest said that the US would not take such a move off its options list.

"I certainly don’t have anything to announce at this point. But obviously the United States does have significant sanctions in place against North Korea because of their repeated violation of their international obligations when it comes to their ballistic missile program," he said. "And I would not in any way take off the table the idea of additional sanctions."

With regard to the planned deployment of a Terminal High Altitude Area Defense system in South Korea, he reaffirmed that it is only aimed at countering the threat from the North, saying that there is no reason for China to be concerned about the decision.

South Korea and the US announced in July that a THAAD battery would be deployed on the peninsula despite strong opposition from China, which is worried that the system could hurt its strategic security interest.

"We've made clear that this is a defense missile -- a ballistic missile defense system that would be oriented toward the threat in North Korea, and that's why we don't believe it should be a subject of concern of the Chinese," he said. (Yonhap)