North Korea has fired three ballistic missiles, Seoul's military said, hours after Pyongyang announced the country's first cases of Covid-19.

"Our military detected around 18.29 (9.29am Irish time) that three short-range ballistic missiles were fired from Sunan area in Pyongyang," South Korea's Joint Chief of Staff told AFP, adding the missiles were fired toward the Sea of Japan.

Japan's defence ministry also confirmed the launch and the country's coastguard said that it had issued a safety warning to ships that "North Korea appears to have launched a missile."

The launch is likely the nuclear-armed country's 16th weapons test so far this year.

The test comes two days after South Korea's new President Yoon Suk-yeol was inaugurated.

North Korea also conducted what Seoul and Tokyo said were two separate ballistic missile tests in the week before the inauguration, although Pyongyang did not confirm the launches.

Despite biting international sanctions over its weapons programmes, North Korea has dramatically ramped up testing this year while ignoring the United States' offers of talks.

Earlier, North Korea confirmed its first Covid-18 infections and declared a "serious emergency", with leader Kim Jong-un appearing in a mask on television for the first time to order nationwide lockdowns.