SEOUL — South Korean and US troops began naval drills Monday in a show of force partly directed at North Korea amid signs that Pyongyang will follow through on a threat to conduct its third atomic test.

The region has also seen a boost in diplomatic activity since last month, when North Korea announced it would conduct a nuclear test to protest UN Security Council sanctions toughened after a satellite launch in December that the United States and others say was a disguised test of banned missile technology.