The United States called North Korea's firing of around 100 artillery rounds into South Korean waters on Monday,. as dangerous and provocative, adding that Pyongyang's threats will only work to futher isolate the regime.
"We remain steadfast in our commitment to the defense of our allies and remain in close coordination with both the Republic of Korea and Japan."
U.S. State Department deputy spokeswoman Marie Harf said she was concerned about North Korea's "deliberate decision to further escalate tensions."
"They can choose to further escalate or they can choose to come in line with international obligations and rejoin the international community. Unfortunately, what we have seen, recently particularly, is the former."
U.S. Defense Secretary Chuck Hagel urged North Korea to discontinue such provocations.
He said he would discuss the issue with his Chinese counterpart during a trip to Beijing next week.
China's Foreign Ministry spokesman Hong Lei joined in on expressing concern over the rising tensions on the Korean peninsula and called for restraint from both sides.
"All sides should remain calm and not do anything to worsen tensions. The two should work to protect peace and stability on the Korean peninsula."
The two Koreas on Monday afternoon exchanged fires across the disputed western sea border, known as the Northern Limit Line.
Pyongyang has been raising its levels of threats in recent days after the United Nations condemned its mid-range missile launches last week, and against a backdrop of joint military drills between South Korea and the United States.
Yoo Li-an, Arirang News.