South Korea said Monday it was "closely watching" for any signs of an imminent North Korean missile test after Pyongyang reportedly issued a no-sail zone off its east coast.

The South's Yonhap news agency had quoted a government source on Sunday as saying the North had imposed the no-navigation zone off the coastal city of Wonsan from November 11 to December 7.

The Defence Ministry in Seoul said no formal notification had been filed with South Korea or the International Maritime Organisation, suggesting that the advisory was for domestic shipping only.

The Japanese government also said it had received no official warning.

Similar advisories in the past have not always been followed by a missile test, but Seoul said it was remaining vigilant.

"We are closely watching the situation ... and closely monitoring related factors from a military standpoint," a defence ministry spokesman said, without elaborating.

The isolated, nuclear-armed North regularly test-fires short-range missiles into the East Sea as part of scheduled military drills, or as a show of force at times of heightened tensions.

The North is banned from tests using any ballistic missile technology under UN Security Council resolutions.

The North claims it has developed long-range missiles capable of hitting the US mainland, but many experts say Pyongyang is still years away from obtaining a credible ICBM capability.