SEOUL (Reuters) - North Korea has fired two surface-to-air missiles off its west coast, South Korea said on Thursday, with the latest in a string of short-range firings by the North coming during a visit to the region by the United States defence secretary.

"The range of these is not that long," Defence Ministry spokesman Kim Min Seok told a briefing, referring to the surface-to-air missiles fired on Tuesday. "The North occasionally fires these," he said.

US Secretary of Defence Ashton Carter was in Japan on Wednesday and was due to travel to South Korea on Thursday, where he is expected to discuss a response to North Korea's growing missile and nuclear threat.

North Korea fired four short-range missiles off its west coast on Friday from a major missile station used previously to launch a long-range rocket that is believed to have put an object into space.