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.

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.

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.