North Korea has fired three short-range missiles from its east coast, South Korea's defence ministry says, but the purpose of the launches was unknown.

Launches by the North of short-term missiles are not uncommon, but the ministry would not speculate whether Saturday's launches were part of a test or training exercise.

"North Korea fired short-range guided missiles twice in the morning and once in the afternoon off its east coast," an official at the South Korean Ministry of Defence spokesman's office said by telephone.

The official said he would not speculate on whether the missiles were fired as part of a drill or training exercise.

"In case of any provocation, the ministry will keep monitoring the situation and remain on alert," he said.

Al Jazeera's Harry Fawcett in Seoul quoted a government spokesperson who would not be named as saying it was probably a KNO2-type missile, that is a short-range guided ballistic missile - the shortest of North Korea's arsenal.

The missile travelled in a northeasterly direction, he said.

"That's interesting because it takes the missiles away not only from South Korean territory but also from Japanese territory."

The ministry said the country had reinforced monitoring and was maintaining a high-level of readiness to deal with any risky developments.

A Japanese government source, quoted by Kyodo news agency, noted the three launches, but said none of the missiles landed in Japan's territorial waters.

'Retaliation unlikely'

Tension on the Korean peninsula has subsided in the past month after running high for several weeks following the imposition of tougher UN sanctions against North Korea following its third nuclear test in February.

The North had for weeks issued nearly daily warnings of impending nuclear war with the South and the US.

Our correspondent said that this could be seen as a way of backing up some of the rhetoric, but doing so in a relatively conservative manner.

"These sorts of short range tests don't attract the same kind of approach that longer range or nuclear tests carry with them and they were fired north easterly. So they are likely not to provoke any sort of retaliatory measure from South Korea or Japan."

North Korea conducts regular launches of its Scud short-range missiles, which can hit targets in South Korea.

It conducted a successful launch of a long-range missile last December, saying it put a weather satellite into orbit.

The US and its allies denounced the launch as a test of technology that could one day deliver a nuclear warhead.

During the weeks of high tension, South Korea reported that the North had moved missile launchers into place on its east coast for a possible launch of a medium-range Musudan missile.

The Musudan has a range of 3,500km, putting Japan in range and possibly the US South Pacific island of Guam.