SEOUL—Chinese and Russian warplanes have increasingly nosed around and veered into South Korea’s airspace, conducting close patrols that allow Beijing and Moscow to test the air defenses of the U.S. and its allies in the region.

The aerial campaigns come as Beijing vows to strengthen its military alliance with Moscow, heightening tensions across the Asia-Pacific region as the U.S. and China jockey for power there.

The Korean Peninsula is once again providing a convenient stage for military provocations, as it did during the Cold War.

The Chinese and Russian jets fly near the coastlines of South Korea, a key Washington ally that hosts the largest U.S. overseas military base as well as a U.S. missile-defense system that has drawn Beijing’s ire. While many of the air maneuvers don’t violate international law, according to Seoul’s military, they challenge Washington’s patience.

It was much less common for Beijing or Moscow to conduct such flights earlier this decade, South Korean officials say, adding the uptick that began in recent years appears to reflect a marked policy change for both countries.