BEIJING (REUTERS) - A Chinese diplomat has met ambassadors from the United States and both Koreas to express "serious concern" over the situation on the Korean peninsula, China's Foreign Ministry said on Wednesday.

"Yesterday afternoon, Deputy Foreign Minister Zhang Yesui met the ambassadors of North and South Korea and the United States and expressed serious concern about the present situation on the peninsula," Chinese Foreign Ministry spokesman Hong Lei told reporters at a daily briefing.

"In the present situation, China believes all sides must remain calm and exercise restraint and not take actions which are mutually provocative and must certainly not take actions which will worsen the situation," he said.

North Korea has threatened a nuclear strike on the US and missile attacks on its Pacific bases after new United Nations sanctions were imposed for the country's third nuclear weapons test in February. It has also said it was in a "state of war" with South Korea. In response, the US has bolstered its forces in the region. Extra deployments have also coincided with annual military exercises with South Korea, which North Korea says are a prelude to an invasion.

North Korea on Wednesday closed access to a joint factory zone that earns US$2 billion (S$2.48 billion) a year in trade for the impoverished state. China hopes all sides can resolve that issue through talks, Mr Hong said.

China has long been accustomed to living with the North as an unpredictable neighbour, which acts as a bulwark against the US. But the unusual move to meet all three ambassadors suggests China is growing uneasy with the tension on its doorstep.