United Nations chief Ban Ki-moon says he is "deeply concerned" by Tuesday's deadly exchange of artillery fire between North and South Korea.
The U.N. secretary-general added his response to those of major world powers, calling the exchange one of the gravest incidents since the end of the Korean War.
The United States has condemned North Korea for attacking the South and demanded that it halt its "belligerent action." Washington also renewed its commitment to defend South Korea from any attack.
China, a North Korean ally, said both Koreas needed to do more to maintain peace between them, and called for an urgent resumption of six-party talks on North Korea's nuclear programs.
Japan called the incident "unforgivable" and said it demands an immediate end to such actions.
Russia, Britain and Germany also commented, expressing varying degrees of alarm.
Russian Foreign Minister Sergei Lavrov said he sees a "colossal danger" that the fighting will escalate, while British Foreign Secretary William Hague condemned Pyongyang for its "unprovoked attack."
German Foreign Minister Guido Westerwelle said he is "very worried" by the artillery exchange.
North Korea fired more than 50 shells into a populated border island while South Korean troops were conducting a military exercise, killing two soldiers and wounding both soldiers and civilians.
Pyongyang said the strikes were made in response to South Korean fire - presumably during the military drill. The South said its troops were firing away from the North Korean mainland.
Some information for this report was provided by AP, AFP and Reuters.