Iranian President Hassan Rouhani warned on Saturday of retaliatory measures against the United States if Washington caused problems for tankers carrying Iranian fuel to Venezuela, the semi-official news agency Mehr reported.

"If our tankers in the Caribbean or anywhere in the world face trouble caused by the Americans, they (the U.S.) will also be in trouble," Rouhani said in a telephone conversation with Qatar's Emir, Mehr reported.

A flotilla of five tankers carrying Iranian fuel for gasoline-starved Venezuela is approaching the Caribbean. Venezuelan state television reported on Saturday that the first tanker would reach the country's waters at 7 p.m. local time.

TankerTrackers.com, a service that tracks oil shipments and storage, estimated that the lead vessel in the flotilla, the Fortune, would arrive in Venezuela's exclusive economic zone (EEZ) between 9 and 11 p.m. local time, according to time and speed calculations.

"Iran will never initiate a conflict," Rouhani said. "We hope that the Americans will not commit an error."

The tankers are carrying around 1.53 million barrels of gasoline and alkylate to Venezuela, according to both governments, sources and calculations made by TankerTrackers.com based on the vessels' draft levels.

The shipments have caused a diplomatic standoff between Iran and Venezuela and the United States as both nations are under U.S. sanctions. Washington is considering measures in response, according to a senior U.S. official, who did not elaborate on any options being weighed.

The United States recently beefed up its naval presence in the Caribbean for what it said was an expanded anti-drug operation. But a Pentagon spokesman, Jonathan Hoffman, said on Thursday he was not aware of any operations related to the Iranian cargoes.

Venezuela's defense minister has said its military will escort the Iranian tankers once they reach the nation's EEZ.