The firing of the missiles, reported to have a range of up to 500 kilometres (312 miles), follow a series of missile launches earlier this week by the isolated Stalinist state.

"North Korea has test-fired three missiles so far this morning," said a defence ministry spokeswoman. The first two missiles were launched between 08:00 am and 08:30 am (2300-2330 GMT Friday) and the third one was fired at 10:45 am, she said.

She said all three missiles were believed to have been launched from Kitdaeryong Base, near the eastern port of Wonsan, into the East Sea (Sea of Japan).

"The military, on the basis of a strong joint defence alliance with the United States, is fully prepared to fend off any threats or provocations by the North," the Joint Chiefs of Staff said in a statement earlier on Friday.

"All the missiles fired (today) are believed to have a range of between 400 and 500 km," an unidentified government official told Yonhap news agency, allowing the North to strike almost anywhere in South Korea. Yonhap said the first two missiles were believed to be Scuds.

North Korea fired four short-range missiles on Thursday into the Sea of Japan but the range of those missiles was estimated to be only around 120 km. "The Scuds fired today impose a greater security threat to us because of their longer ranges," the official was quoted as saying by Yonhap.

"Thursday's missile tests were apparently made as part of a military drill but today's launches, which came on the eve of the US Independence Day, are believed to be aimed at political purposes," the official said.

Japan condemned the latest missile launches as a "serious act of provocation." "It is a serious act of provocation against the security of neighbouring countries, including our country," Chief Cabinet Secretary Takeo Kawamura was quoted by Jiji Press as saying. The Japanese government's top spokesman said the act was also against a UN Security Council resolution, according to Jiji.

Analysts said the North was flexing its military muscle amid tough international sanctions imposed on Pyongyang over its nuclear weapons and missile programmes.

Baek Seung-Joo of the Korea Institute for Defence Analyses said the North test-fires missiles three to four times each year to improve technology and maintain missile exports.