The vessels departed last month from Iran’s southern port of Bandar Abbas, said Adm. Habibollah Sayyari, Iran’s deputy army chief. He described their mission as the Iranian navy’s longest and most challenging voyage yet, without elaborating.

Iranian state TV released a short clip of the destroyer cruising through the Atlantic’s rough seas. The video likely was shot from the Makran, a converted commercial oil tanker with a mobile launch platform for helicopters.

“The Navy is improving its seafaring capacity and proving its long-term durability in unfavorable seas and the Atlantic’s unfavorable weather conditions,” Sayyari said, adding that the warships would not call at any country’s port during the mission.

Images from Maxar Technologies dated April 28 appear to show seven Iranian fast-attack craft typically associated with its paramilitary Revolutionary Guard on the deck of the Makran. Satellite images from Planet Labs Inc. suggest it left a port at Bandar Abbas sometime after April 29. It wasn’t immediately clear where the Makran and the destroyer are now.

The website Politico first reported in late May, citing anonymous officials, that the ships’ final destination may be Venezuela. Iran maintains close ties to Venezuelan President Nicolas Maduro, and has shipped gasoline and other products to the country amid a U.S. sanctions campaign targeting fuel-starved Caracas. Venezuela is believed to have paid Iran, under U.S. sanctions of its own, for the shipments.

A top aide to Maduro has denied press reports that the ships will dock there. The official spoke on the condition of anonymity to discuss sensitive geopolitical issues.

During a news conference May 31, Iranian Foreign Ministry spokesman Saeed Khatibzadeh declined to say where the Makran was going.

“Iran is always present in international waters and it has this right based on international law and it can be present in international waters,” he said. “No country is able to violate this right, and I warn that no one makes miscalculations. Those who sit in glass houses should be careful.”

The fast-attack craft aboard the Makran are the type that the Guard uses in its tense encounters with U.S. warships in the Persian Gulf and its narrow mouth, the Strait of Hormuz. It’s not immediately clear what Venezuela’s plans would be for those ships.