The South Korean and US militaries on Monday started their first major combined training since North Korea's test-firing of two long-range missiles and threat to shoot ballistic missiles toward Guam.

The Ulchi Freedom Guardian exercise will run through next Thursday, focusing on a "tailored joint deterrence strategy" based on the Operational Plan 5015 of the Combined Forces Command, officials said.

In an unusual gesture, three top US military commanders -- Pacific Command chief Adm. Harry Harris, Strategic Command head Gen. John Hyten and Missile Defense Agency Director Lt. Gen. Samuel Greaves -- will observe the command-post exercise in person.

It involves tens of thousands of South Korean soldiers. In a press statement, the Ministry of National Defense did not specify the number. Ministry officials said the size is similar to that of last year, roughly 50,000.

The US has reduced the number of its participating service members to 17,500 from 25,000, according to the Pentagon. The troops include some 3,000 coming from off-peninsula.

Seven other member states of the United Nations Command -- Australia, Canada, Colombia, Denmark, New Zealand, the Netherlands and Britain -- are also taking part in it.

There's no report of the US dispatching such high-profile weapons as aircraft carriers, nuclear-powered submarines or strategic bombers to the training.

"UFG is computer simulated defensive exercise designed to enhance readiness, protect the region and maintain stability on the Korean peninsula," it said.

Officials from the two countries denied speculation that links the decrease in the number of US participants to an effort to cool down tensions on the peninsula.

Defense officials here pointed out that UFG is largely a war game, rather than a field exercise like Foal Eagle held in the spring.

They said the number of participating troops can slightly change depending on the emphasis of the training events each a year.

This year's UFG comes amid brewing speculation about the future of the Seoul-Washington alliance as the allies are under growing pressure to downsize their regular combined military training, which Pyongyang views as a rehearsal for a northern invasion.

Last week, Stephen K. Bannon, a key aide to President Donald Trump, openly raised the possibility of a deal to withdraw American troops from South Korea in return for a verifiable freeze in the North's nuclear program. He was later dismissed from the White House chief strategist post.

On a recent trip to Northeast Asia, Gen. Joseph F. Dunford, chairman of the Joint Chiefs of Staff, refuted the view that a troop pullout is being considered. He emphasized the US has a "long-term alliance commitment" to the defense of South Korea.

The North launched two intercontinental ballistic missiles in July and announced a plan for a strike near Guam with four mid-range missiles.

Trump issued a warning of unprecedented "fire and fury."

The two sides are at an uneasy truce in the war of words after the North said its leader Kim Jong-un has ordered his armed forces to hold off on the Guam attack scheme and see how the US acts.

The allies are on alert for possible North Korean provocations aimed at protesting the practice and driving a wedge between them.

Last year, the North fired a submarine-launched ballistic missile (SLBM) in the East Sea two days after the opening of UFG. It also launched three Scud missiles on the last day of the training followed by a nuclear test four days later.

President Moon Jae-in urged the North to refrain from making additional provocative acts on the pretext of the drills being defensive in nature.

North Korea watchers said the North's attitude during and shortly after the UFG period will be a critical factor in the security condition here in the coming months, with the South and the US trying to bring it to the bargaining table. The North will celebrate its founding anniversary on Sept. 9. (Yonhap)