Mexican President Claudia Sheinbaum on Tuesday ruled out allowing U.S. strikes against cartels on Mexican soil, a day after Trump said he was willing to do whatever it takes to stop drugs entering the U.S. Meanwhile, Mexican and American diplomats were trying to sort out what may have been an actual U.S. incursion.

On Monday, men arrived in a boat at a beach in northeast Mexico and installed some signs signaling land that the U.S. Department of Defense considered restricted.

Mexico’s Foreign Affairs Ministry said late Monday that the country’s navy had removed the signs, which appeared to be on Mexican territory. “The origin of the signs and their placement on national territory were unclear,” the ministry said in a statement.