North Korea has told its merchant seamen not to venture into several sectors of the Yellow Sea for the next two months, raising fears of missile tests and potential naval conflict near the disputed maritime border, South Korean officials said yesterday.

The communist state has threatened to take “self- defence measures” if the United Nations Security Council punishes its test of an atomic bomb last week. In addition to raising the prospect of conflict at sea, US officials say Pyongyang appears to be gearing up to test-fire its second long-range missile since April 5.

South Korean officials said they often intercepted messages that declared certain zones out of bounds because of military training but added it was very rare for exclusion areas to last for two months. The two Koreas fought naval battles in the Yellow Sea in 1999 and 2002, with dozens of casualties. North Korea no longer recognises the maritime border there.