Iran has sent an official warning to the U.S. against making threats to Iranian fuel tankers bound for Venezuela, the state-run Islamic Republic News Agency reported.

On Sunday Iran’s Ministry of Foreign Affairs summoned the Swiss envoy Markus Leitner, who represents U.S. interests in the Islamic Republic, to warn that any measures taken against Iranian fuel tankers will be “met with a swift and decisive response,” IRNA reported, citing the foreign ministry.

The summons comes after reports that Iran is helping Venezuela, which is also subject to crippling U.S. sanctions, ease a major gasoline shortage. Iran’s Foreign Minister, Mohammad Javad Zarif, also sent a letter to UN Secretary-General Antonio Guterres, warning against the U.S. taking action in the Caribbean in order to interfere with Iranian fuel deliveries to Venezuela, IRNA said.

At least five chemical and oil products tankers entered the Mediterranean Sea in the past 10 days having previously been to Iran, according to tanker tracking data compiled by Bloomberg last week.

Deputy Foreign Minister Abbas Araghchi told Markus Leitner on Sunday that Iran-Venezuela trade relations are legal and legitimate and that U.S. “bullying and coercive actions” amounted to “piracy and a threat to freedom of shipping,” according to IRNA.

With its economy strangled by U.S. sanctions, Venezuela has turned to Iran, an ally whose economy is also subject to severe U.S. sanctions, to help relieve a major fuel shortage with blending components, spare parts and technicians.