The South Korean corvette the Cheonan was sunk off the peninsula's west coast in March, killing 46 sailors. South Korea has blamed the North for sinking the Cheonan with a torpedo fired from a submarine, a charge the North denies.
The United States and South Korea have since said they will hold a joint anti-submarine drill.
Some news reports, including one in the Washington Post newspaper, have said Pentagon officials are considering sending an aircraft carrier to take part in the exercise in the Yellow Sea off South Korea's western coast.
Seoul says an international inquiry showed there was no doubt North Korea sank the Cheonan but Beijing, North Korea's only real ally and benefactor, has reacted much more cautiously.
Chinese Foreign Ministry spokesman Qin Gang said he had seen the reports about the U.S. aircraft carrier joining the drill.
"We're extremely concerned about these reports and will closely follow developments," he told a news conference.
He said China was worried the show of naval force could unsettle the region. "Under current circumstances, all the parties concerned should exercise calm and restraint and do nothing to escalate tensions and harm the interests of countries in this region," Qin said.
North Korea said the planned exercises were part of a "dangerous scheme to disturb the peace," the state news agency KCNA quoted a major daily as saying on Tuesday.
"The situation of the Korean Peninsula is now at the crucial phase in which an all-out war may break out any moment. Therefore, the reckless joint military exercises projected under such a situation cannot be construed otherwise than a scenario to ignite another Korean war."
China has resisted calls from Seoul, Washington and Tokyo to join in condemning Pyongyang over the Cheonan sinking, instead saying it needed to assess the competing claims.
Qin repeated that position on Tuesday.
Beijing has also been irked by U.S. navy ships engaging in surveillance in waters close to China's southern coast.
(Reporting by Chris Buckley in Beijing and John Ruwitch in Seoul; Editing by Ron Popeski)
