The United States is warning that a new round of artillery drills by North Korea is dangerous and must stop.
U.S. Defense Secretary Chuck Hagel gave the stern response during a briefing with reporters Monday.
"The provocation that the North Koreans have once again engaged in is dangerous and it needs to stop.''
Hagel said the subject obviously will be something he discusses with his Chinese counterpart during his upcoming trip to Beijing.
China has said it is concerned by Pyongyang's actions, but has called for restraint from both North and South Korea.
North Korea fired more than 100 artillery rounds into South Korean waters Monday, apparently in reaction to military exercises by the U.S. and South Korea.
White House spokesman Jay Carney echoed the U.S. defense secretary's comments, calling North Korea's actions dangerous and provocative.
"The DPRK's continued threats and provocations aggravate tensions and further North Korea's isolation. We remain steadfast in our commitment to the defense of our allies, and we remain in close coordination with both the Republic of Korea and Japan."
Seoul responded to Pyongyang's actions Monday by immediately shelling North Korean waters. Officials say no shells hit land on either side of the western sea boundary and no one was injured. South Korean officials said some residents of border islands were evacuated to shelters as a precaution.
U.S. General Paul Kennedy says the joint exercises taking place about 360 kilometers north of Seoul have no political objectives.
Pyongyang routinely calls military drills involving South Korea and the U.S. a rehearsal for invasion.
The North in recent weeks has increased threatening rhetoric and conducted a series of rocket and ballistic missile launches into waters off the east coast of the Korean peninsula.
Pyongyang threatened on Sunday to conduct what it called "a new form of nuclear test" after the U.N. Security Council condemned the North's recent ballistic missile launches.