Seoul, Aug 21 (IANS): The South Korean and US militaries on Monday began their first major annual military exercise amid threats from North Korea.

The Ulchi Freedom Guardian (UFG) exercise will conclude on August 31. It is 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 U. military commanders -- Pacific Command chief Admiral Harry Harris, Strategic Command head Gen. John Hyten and Missile Defence Agency Director Lt. Gen. Samuel Greaves -- will observe the command-post exercise in person, reports Yonhap News Agency.

It involves tens of thousands of South Korean soldiers. In a press statement, the Ministry of National Defence 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.

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

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.

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

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

US President Donald Trump responded with a warning of unprecedented "fire and fury".