South Korea detected "two short-range ballistic missiles fired by North Korea from the Sunan area towards the East Sea at around 11:43 to 11:53 today", the Joint Chiefs of Staff said, referring to the body of water also known as the Sea of Japan.

"Our military has strengthened surveillance and vigilance in preparation for further launches, while maintaining full readiness by closely cooperating with the US."

The Japanese coast guard said two ballistic missiles were launched.

Japan's top government spokesman Hirokazu Matsuno told reporters that the missiles "appear to have fallen outside Japan's exclusive economic zone but we are analysing details".

Kim is in Russia with his top military officials and is set to meet Putin on Wednesday for talks focused on defence cooperation.

Russia is eager for North Korea's stockpile of artillery shells, likely for use in Ukraine, while Pyongyang is looking for help in upgrading its Soviet-era equipment, especially for its air force and navy, experts say.

"Fascinating: a launch without Kim Jong (Un) in the country. A first," US-based analyst Ankit Panda wrote on X.

"Starting in 2019, KJU stopped attending every single publicized launch (and sometimes his presence was deliberately obscured)," he wrote.

"There's precedent for launches without Kim, but not without Kim in-country."

North Korea has conducted a string of banned weapons tests so far this year, the last one involving two short-range ballistic missiles on August 30.

It also last month failed in its second attempt to put a spy satellite into orbit.

South Korea and the United States have ramped up defence cooperation in response, staging joint exercises as well as naval drills with Japan.

Relations between the two Koreas are at their lowest point in years, and diplomacy is stalled after failed attempts to discuss Pyongyang's denuclearisation.