China deplored rising tension on the Korean peninsula on Sunday but said its embassy was operating normally in the North Korean capital and it asked authorities there to ensure its diplomats and other citizens were kept safe.

North Korea, angry about new U.N. sanctions imposed for its third nuclear weapon test in February, has made increasingly strident warnings of an imminent war with South Korea and the United States.

The United States, keen to avoid action that could provoke the North, on Saturday postponed a long-scheduled missile test in California.

The North told diplomats on Friday to consider leaving Pyongyang because of the tension, but diplomatic missions appeared to view the appeal as more rhetoric and staff have stayed put.

South Korea said it was ready to counter any move, including a missile launch, that the North's unpredictable leaders might make.

China is reclusive North Korea's sole major diplomatic and financial backer, but its official statements have reflected an increasing impatience with the action of North Korean authorities under 30-year-old leader Kim Jong-un.

"At present, tensions on the Korean peninsula are rising unceasingly, and China expresses grave concern about this," China's Foreign Ministry said in a statement on its website (www.mfa.gov.cn).

China, it said, had asked North Korea "to earnestly ensure the safety of Chinese diplomats in North Korea, in accordance with the Vienna Convention and international laws and norms".

The ministry said China's embassy was "understood" to be operating normally in Pyongyang. China would "protect the legal rights and safety of Chinese citizens and Chinese-invested organizations in North Korea". It did not elaborate.

Chinese President Xi Jinping, addressing a forum on the southern island of Hainan, appeared to refer to the tension when he said no country "should be allowed to throw a region and even the whole world into chaos for selfish gain".

Stability in Asia, he said, "now faces new challenges, as hot spot issues keep emerging and both traditional and non-traditional security threats exist".

Australian Prime Minister Julia Gillard told the same forum that avoiding conflict on the Korean peninsula was vital.

"There, any aggression is a threat to the interests of every country in the region," she said.

"I do welcome the growing co-operation of all regional governments to prevent conflict on the Korean peninsula and to counter North Korean aggression ... Asia must be a region of sustainable security in which habits of co-operation are the norm."

In Washington, a defense official said a test of the Minuteman III intercontinental missile, scheduled to take place at the Vandenberg Air Force Base in California, would be postponed.

"This is the logical, prudent and responsible course of action to take," the official said, speaking on condition of anonymity.

He said the test had been unconnected to "anything related to North Korea" and added that another test launch could be expected next month. The United States remained fully prepared to respond to any North Korean threat, the official said.

Missile launch "could occur"

The South Korean president's office said the country had a "firm military readiness" for any eventuality. It described as "planned behavior" the North's call for South Korean workers to leave the Kaesong joint industrial park, just inside North Korea, and for diplomats to evacuate Pyongyang by April 10.

"Ahead of that time, a situation like a North Korean missile launch could occur," Kim Haing, a spokeswoman for the presidential Blue House, quoted the chief of the National Security office as saying.

"As of now, there are no signs of all-out war, but if a local conflict breaks out, North Korea should be aware that it will pay the price."

Kim Jong-un is the third member of his dynasty to rule North Korea. He took over in December 2011 after the death of his father, Kim Jong-il, who staged confrontations with South Korea and the United States throughout his 17-year rule.

North Korean anger over the sanctions following its nuclear weapon test has been compounded by joint U.S.-South Korean military exercises that began on March 1.

The North has always condemned the exercises but it has been especially vitriolic this year as the United States dispatched B-2 bombers from their home bases to stage mock runs.

North Korean television provided little evidence of tension on Sunday, with newscasts showing old footage of Kim visiting military units. Nor was there any tension in Seoul, with residents strolling in the city centre on a chilly spring day.

South Korean media said on Friday the North had moved two medium-range missiles to the country's east coast, but there has been no confirmation of such a move. That prompted the White House to say that Washington would "not be surprised" if the North staged another missile test.

North Korea has not shut down a symbol of cooperation with the South, the Kaesong industrial zone. But last week, it prevented South Koreans from entering the complex and 94 returned home on Saturday, the Unification Ministry said, leaving a further 518 there.