North Korea test-fired two short-range missiles 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 launches were unrelated to the announcement that Kim had died Saturday of a heart attack.

"The missiles are estimated to have a range of about 120 kilometres (72 miles)," the official said on condition of anonymity.

"We believe the North test-fired the missiles to try to improve their capabilities and range. We don't see this as more than test-firing."

Seoul's defence ministry declined to confirm the reported launches.

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.