Two Iranian warships have joined a cargo ship that Tehran says is carrying supplies to Yemen.

US Army Col. Steve Warren said the ships linked up overnight Monday, setting up a possible conflict as the US insists the supplies go to Djibouti, where the United Nations is coordinating humanitarian aid for Yemen.

A US official said there’s no evidence the Iranian ship is carrying lethal or military aid. The official spoke on condition of anonymity because the official was not authorized to discuss the matter publicly.

The vessel, the Iran Shahed, is carrying nearly 2,800 tons of aid including flour, rice, canned food, medical supplies and bottled water, all urgently needed in the conflict-wracked and impoverished state, according to Iran. It is expected to reach port on Thursday, and entered the Gulf of Aden on Sunday.

Passengers on the Iran Shahed include doctors, anti-war activists from the United States, France and Germany, and journalists, according to the Tasnim news agency.

The vessel’s captain, Masoud Qazi Mir-Saeed, said that if weather remained fair it should dock in rebel-controlled Hodeida on Thursday.

The Pentagon had urged the ship to divert to Djibouti — where the UN has an aid hub across the narrow strait that separates Yemen from the Horn of Africa — to prove its cargo was humanitarian.

The dispute raised concerns of a potential confrontation between the US and Iran in the vital sea lane which links the Gulf and the Indian Ocean with the Mediterranean through the Suez Canal.

The US has accused Iran of militarily backing the Yemeni rebels, known as Houthis. The US has five warships in the western Gulf of Aden.

Iran said last week that it would protect the supply ship, and threatened war if it was attacked.

A Marshall Islands-flagged cargo ship was recently detained by Iran for more than a week as it traveled through the Strait of Hormuz.

Iran claimed the Danish shipping company that chartered the MV Maersk Tigris owed money to an Iranian firm. Last week, an Iranian naval patrol in the Persian Gulf fired on a Singapore-flagged commercial tanker, causing no injuries.