SEOUL, Aug 19 (Reuters) - South Korea and the United States kicked off annual summertime military exercises on Monday, seeking to boost their joint readiness to fend off North Korea's weapons and cyber threats.

The Ulchi Freedom Shield exercises, set to end on Aug. 29, come as North Korea races to advance its nuclear and missile programmes and tries to launch reconnaissance satellites.

The drills will reflect "realistic threats" across all domains, including the North's missile threats but also GPS jamming, cyberattacks and other lessons learned from recent incidents, the two countries' militaries have said.

South Korea will separately carry out the simultaneous government-led Ulchi civil defence drills, under the scenario of a nuclear attack by the North, officials said.

South Korean President Yoon Suk Yeol urged thorough readiness posture against North Korea, calling it "the most reckless and irrational country in the world.

"As seen in Ukraine and the conflicts in the Middle East, war can break out at any time," he told a cabinet meeting.

"The nature of war has also changed from the past, being carried out in a hybrid form that mixes regular, irregular and cyber warfare, and even public opinion and psychological warfare using fake news."

About 19,000 South Korean troops will take part, similar to last year, with 48 rounds of combined field training, including field manoeuvre, live fire and amphibious exercises to be held, Yoon said.

Pyongyang has long denounced the allies for stoking tensions with military drills, calling them rehearsals for a nuclear war.

Seoul and Washington say the exercises are defensive and a response to the North's threats.

Photo from Reuters