April 15 (UPI) -- The U.S. military killed four men on Tuesday in a strike on a suspected drug-trafficking boat in the Pacific, its fourth vessel attack in four days.

Since Saturday, the United States has killed at least 11 people in strikes targeting alleged drug-trafficking boats under President Donald Trump's anti-narcotics operation, which rights groups and legal experts have criticized as unlawful.

With the four killed Tuesday, the known death toll of the anti-drug-trafficking operation, which began in early September, rises to at least 174, according to UPI's tally of available figures.

The U.S. Coast Guard had launched a search for a lone survivor from one of Saturday's strikes. UPI has contacted the Coast Guard for comment on the search.

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