Donald Trump’s latest strategy to ramp up pressure on Iran has been dealt an embarrassing blow after several ships defied his blockade in the Strait of Hormuz.

Multiple Iran-linked tankers—including sanctioned ships and a Chinese-linked tanker—transited the vital waterway within hours of the U.S. Navy’s blockade taking effect.

The news raised immediate questions about the effectiveness of Trump’s plan, which was announced on Sunday after the collapse of U.S.-Iran talks in Islamabad.