The United States and South Korea kicked off their major annual joint military drills on Monday, Seoul said, with new exercises aimed at containing the nuclear-armed North, including fighting cyber attacks.

The Ulchi Freedom Shield exercise runs until August 29, with drills this year set to "reflect realistic threats across all domains", including from North Korean missiles and GPS jamming, Seoul's Joint Chiefs of Staff said.

The exercises will boost the allies' "capability and posture to deter and defend against weapons of mass destruction", the military added.

This year's drills will involve about 19,000 South Korean troops participating by land, sea and air, as well as in the cyber and space domains, according to the defence ministry, which declined to offer details of the US's participation.

Pyongyang has floated thousands of trash-carrying balloons across the border in recent months, and sought to jam South Korean GPS signals as part of its protest against balloons carrying anti-regime propaganda sent northwards by activists in the South.

In Seoul, the city government will be simultaneously conducting civil defence exercises designed to better prepare for any future trash balloons, as well as North Korean drone attacks.

"We are currently facing the most reckless and irrational North Korean provocations and threats in the world," South Korean President Yoon Suk Yeol said at a cabinet meeting Monday, according to his office.

"In recent years, they have not hesitated to launch GPS jamming attacks and make low-grade provocations such as launching trash balloons."

US-South Korea drills typically infuriate the North, which views them as rehearsals for invasion and has frequently conducted weapons tests in retaliation.

On Sunday, North Korean state media slammed the drills, saying they were "dangerous and grave".

Last year, North Korean state media warned the drills could trigger a "thermonuclear war", launching a number of cruise missiles and ballistic missiles in protest.

Kim Myung-soo, head of Seoul's Joint Chiefs of Staff, said last week that Pyongyang was "likely to use the exercise as a pretext to conduct deceitful and blitz provocations".

Kim ordered troops to "closely monitor and analyze the activities of the DMZ" and "retaliate immediately if the enemy provokes".

Washington is Seoul's key security ally and stations about 28,500 troops in South Korea to protect it from its nuclear-armed neighbour.

The two countries have long carried out joint exercises.