The U.S. military conducted another strike on an alleged drug-trafficking boat in the Caribbean, killing four people, part of the broader “Operation Southern Spear” anti-drug campaign.

Since September, U.S. forces have destroyed at least 21 vessels in international waters, though officials have provided little evidence linking them to drug smuggling.

Experts and the U.N. human rights chief have raised concerns about potential violations of international law and possible extrajudicial killings.

The strikes coincide with a U.S. military buildup in the region, including the USS Gerald Ford aircraft carrier, and discussions of potential operations against Venezuela, CBS News has reported.

In response, Venezuela launched a large military exercise, while Colombia’s president halted intelligence sharing with the U.S. over the attacks.