The United States military has launched an intensive spying operation over North Korea, amid reports that the isolated totalitarian state may be about to test fire short range missiles close to its disputed sea border with South Korea.

A US military spokesman confirmed that unspecified monitoring “assets” – probably including spy satellites and high altitude spy planes – have been moved into position amid reports of imminent missile launches, after unusual movements by shipping in the area.

South Korean officials confirmed the sudden disappearance of Chinese fishing boats from an area off the north-west coast of the Korean peninsula, possibly because of a shipping warning in advance of missile firing exercises.

North Korea regularly test fires such short range ship-to-ship missiles which do not, in themselves, pose an immediate threat to its neighbours. But the timing of such a test this week would be provocative, coming as it does after Pyongyang renounced military agreements with the South, and just before Hillary Clinton’s first visit to Asia as US secretary of state.

“We're watching those things closely with all the assets we have,” Marine Major Bradley Gordon, a spokesman for the US Pacific Command, said. “We've got all sorts of sensors all around the area. But I won't say what or where they are.”

Mrs Clinton told the government of North Korea that its recent threats and military posturing are “unacceptable”, and promised to discuss the problem with the leaders of Japan, South Korea, Indonesia and China when she visits them next week on her inaugural overseas trip.

“I know of the continuing concern on the part of the other members of the six-party talks [on North Korean nuclear disarmament] with respect to North Korea's attitude in the last weeks,” she said in Washington. “North Korea has to understand that all of the countries in East Asia have made it clear that its behaviour is viewed as unacceptable”.

Last week South Korean officials revealed signs of North Korean preparation for a far more significant missile test –of a North Korean Taepodong 2 missile, a long range weapon with the potential to cross the Pacific and strike the western reaches of the US. A large cylindrical object was photographed by satellites on a train travelling in the direction of North Korean test firing sites.

The North Koreans fired an earlier version of the Taepodong in 1998. In 2002, a Taepodong 2 broke up soon after being launched, and even a successful launch now would not represent an immediate threat to the US and its allies.

“Since the first time that they launched the missile – it flew for a few minutes before crashing – the range of the Taepodong 2 remains to be seen,” the US defence secretary, Robert Gates, said yesterday in Washington. “So far, it's very short.”

The firing of such a large missile would require several more weeks of preparation. The North Korean government many therefore decide to put pressure on Mrs Clinton by firing its smaller missiles close to its sea border with the South, a disputed line which has seen deadly naval skirmishes in 199 and 2002.

Times Recommends

• Tehran cannot live up to its rhetoric
• The new unicycle that will ‘change the world’
• We cannot wish away Iran’s bomb