More than 200 South Korean and U.S. fighter jets are due to fly around the clock for five days this week in what would mark the largest number of sorties flown in training by the allies, South Korea's air force said on Tuesday.

The drills are part of the annual Ulchi Freedom Shield exercises that are designed to boost the joint readiness of the South Korean and U.S. militaries against threats from North Korea, it said.

Aircraft on defensive patrol flights are being diverted to air combat scenarios under simulated drills involving mock enemy aircraft interdiction and cruise missile defence, it said.

Aircraft from two U.S. fighter wings based in South Korea are taking part, the air force said. The United States stations 28,500 troops in the South for combined defence against nuclear-armed North Korea.

North Korea routinely denounces the annual Ulchi Freedom Shield drills, which began on Monday and will run through to Aug. 29, for stoking tensions on the Korean peninsula, calling them rehearsals for a nuclear war.

While it is trying to modernize its air fleet, the North faces challenges in introducing new aircraft and relies largely on ageing and obsolete combat aircraft, even some old Soviet MiG fighters that were introduced in the 1950s.

Pyongyang has ramped up its tactical warfare capabilities involving short-range missiles and heavy artillery that are aimed at striking the South, after having made dramatic advances in longer-range ballistic missile and nuclear programs.