The first of five Iranian oil tankers headed to Venezuela has arrived, amid tension over the possibility the United States could attempt to halt the deliveries.

“The first Iranian tanker has reached Venezuela’s coasts. Thanks to the Bolivian Armed forces for escorting them,” the Iranian embassy in Venezuela tweeted on Sunday.

Iranian President Hassan Rowhani had on Saturday warned the United States against launching attacks on the country’s oil tankers as they pass through the Caribbean Sea on their way to Venezuela.

“If the Americans cause problems for our oil tankers in the Caribbean Sea, we will also cause problems for them,” Rowhani said in a telephone call with Qatar’s Emir, Tamim bin Hamad al-Thani.

Venezuela, once one of the world’s leading oil producers, has been reduced to energy poverty by US sanctions which have crippled its ability to refine its tapped reserves.

Rowhani stressed that Iran does not want a new conflict with the US, but that the country would reserve the right to defend its interests.

“We therefore hope that the Americans will not make a mistake in this regard,” said the president, according to comments published on his office’s web portal.

The warning comes in the midst of a spat about Iranian fuel shipments to Venezuela. Five Iranian oil tankers are currently on course to Venezuela and are due arrive in late May or early June.

The US has imposed tough economic sanctions on both Iran and Venezuela and could decide to send its navy to prevent the tankers reaching their destination.

In a letter to UN Secretary General Antonio Guterres earlier this week, Iranian Foreign Minister Mohammed Javad Zarif warned that any US attack would be “illegal and a form of piracy.”

Iran has had close ties to Venezuela for many years.