North Korea test-fired a short-range missile off its east coast on Monday, the same day it announced the death of leader Kim Jong-Il, South Korea's Yonhap news agency said.

The agency quoted an unnamed government official as saying the missile launch was unrelated to the announcement that Kim had died Saturday of a heart attack.

"North Korea test-fired a short-range missile this morning... it has been closed monitored by our military authorities," the official was quoted by Yonhap as saying.

Seoul's defence ministry declined to confirm the report.

The missile is believed to have a range of about 120 kilometres (72 miles), he said, adding the North was apparently trying to improve the weapon.

North Korea has been testing its new KN-06 missile, a modified version of the KN-01 and KN-02 ground-to-ground missiles, Yonhap said.

The communist country has frequently conducted short-range missile tests in recent years. South Korean officials say they are part of routine exercises but the tests are sometimes timed to coincide with periods of tension.

South Korea put its military on alert as the North's state television announced at noon that the 69-year-old leader had died.