The US and South Korea on Monday launched a major joint military drill, with President Yoon Suk Yeol calling for bolstering readiness against North Korea's "gray zone provocations and hybrid warfare."

The 11-day Ulchi Freedom Shield (UFS) exercise, an annual affair, is aimed at bolstering joint defense readiness amid "advancing North Korean military threats," Seoul-based Yonhap News reported.

The exercise, which runs through Aug. 29, is being held on the heels of heightened cross-border tensions between South and North Korea.

The drill included computer simulation-based command post exercise, concurrent field training and civil defense drills, according to South Korea's Joint Chiefs of Staff (JCS).

The drill involves around 19,000 South Korean troops, 48 field training events, such as amphibious landing and live-fire drills, up from 38 field events conducted last year.

The number of brigade-level exercises will also increase to 17 this year, compared with four from the previous year.

"The exercise will further strengthen the allies' capability and readiness to respond to any provocations and defend against North Korea's weapons of mass destruction by conducting multi-domain exercises utilizing a variety of assets, including land, sea, air, cyber and space," the JCS said.

Citing an unnamed source, Yonhap News reported that during the exercise, the government-led Ulchi civil defense drill will involve a scenario simulating a "North Korean nuclear attack" for the first time.

During a Cabinet meeting, Yoon said that the joint drill will focus on integrating the allies' capabilities under various crisis scenarios to "deter North Korea's evolving military threats, psychological warfare and cyberattacks."

"We must strengthen our readiness to respond to North Korea's gray zone provocations, such as the spread of false information, fake news and cyberattacks," Yoon said. "Anti-state forces that threaten the free democracy are operating covertly in various places," Yoon was quoted as saying.

He warned that Pyongyang "might seek to create social instability through violence, propaganda and agitation at the early stages of a conflict."

Pyongyang has long slammed the allies' joint exercises as a rehearsal for an invasion against it.

Washington and Seoul, for their part, have rejected the accusation, describing their drills as "defensive in nature."