North Korea has threatened to turn Washington and Seoul into “flames and ashes”, warning of an indiscriminate “pre-emptive nuclear strike of justice” in reaction to the start of US-South Korean military drills. Such threats have been a staple of the North Korean leader, Kim Jong-un, since he took power after his father’s death in December 2011. But they tend to increase when Washington and Seoul stage what they describe as annual defensive springtime war games.

North Korea’s powerful National Defence Commission threatened strikes against targets in South Korea, US bases in the Pacific and the US mainland, saying its enemies “are working with bloodshot eyes to infringe upon the dignity, sovereignty and vital rights” of the country.

The joint U.S. and South Korean military command said it had notified North Korea of “the non-provocative nature of this training” involving about 17,000 American troops and more than 300,000 South Koreans. South Korea’s Defence Ministry said it had seen no sign of any unusual military activity by the North.

Relations between North Korea and Seoul and Washington have worsened since North Korea’s nuclear test in January and a long-range rocket test last month, which outsiders said was a test of banned ballistic missile technology. The United Nations recently imposed strong new sanctions on North Korea , and South Korea has said it will announce new unilateral sanctions on Tuesday.