North Korea test-fired three missiles on the eve of American Independence Day celebrations, South Korea's defence ministry said, further stoking tensions amid an international nuclear standoff.

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.

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