South Korea and the United States began holding a smaller-scale joint air drill on Monday, replacing the much larger Vigilant Ace exercise for a fourth year in a row. 

According to military sources, the allies will conduct the scaled-back exercises dubbed a "Combined Flying Training Event" through Friday. 

Roughly 100 fighter jets from each country are said to be taking part, including South Korea’s F-15K and KF-16 jets, as well as the U.S.’ F-16s. 

South Korea and the U.S. had conducted the Vigilant Ace exercise every year in December but scaled them back since 2018 when a dialogue mood was created between the two Koreas and between the U.S. and North Korea. 

Some observers have criticized Seoul and Washington for excessively scaling back the exercises and conducting them in a private manner.