A senior Iranian military official warned that any effort to board a Yemen-bound ship – supposedly filled with aid supplies – would “spark a fire,” amid speculation that Tehran is using the shipment to try to provoke an incident.
One U.S. official told Fox News the Iranian ship has media aboard.
"Iran is begging for us to board the ship. This is all for show," he said.
The lone Iranian cargo ship left Monday and continues to sail in the direction of Yemen. The ship is traveling just a few weeks after a convoy of Iranian ships, carrying weapons, was forced to reverse course after the U.S. Navy sent an aircraft carrier to trail the vessels.
This time, the Iranians claim the ship is carrying only relief supplies to Yemeni citizens, and so far U.S. military officials have not challenged these claims.
"The Iranians were not subtle last time. They had rocket launchers and other weapons on the decks of their ships, but we are not seeing the same thing now," a U.S. Navy official told Fox News.
However, U.S. officials cautioned that Iran might be trying to stage some sort of “stunt” all the same.
Adding to the tension, a top Iranian official warned the Saudi-led coalition targeting Yemeni rebels not to intervene in the shipment.
"I bluntly declare that the self-restraint of Islamic Republic of Iran is not limitless," Gen. Masoud Jazayeri, the deputy chief of staff, told Iran's Arabic-language Al-Alam state TV late Tuesday.
"Both Saudi Arabia and its novice rulers, as well as the Americans and others, should be mindful that if they cause trouble for the Islamic Republic with regard to sending humanitarian aid to regional countries, it will spark a fire, the putting out of which would definitely be out of their hands."
Iran says the ship, which departed Monday, is carrying food, medicine, tents and blankets, as well as reporters, rescue workers and peace activists. It says the ship is expected to arrive at Yemen's port city of Hodeida next week.
Iran's navy said Tuesday it will protect the ship, and on Wednesday Foreign Ministry spokeswoman Marzieh Afkham said Iran would not permit any country involved in the Yemen war to inspect its cargo.
In Washington, U.S. Army Col. Steve Warren said the American military is monitoring the cargo ship. He also said Iran should send the ship to Djibouti, where humanitarian efforts for Yemen are being coordinated.
The U.S. has discouraged aid shipments outside that U.N. process.
The U.S., which supports the coalition, and Saudi Arabia have accused Iran of arming the Yemeni rebels, known as Houthis. Iran supports the rebels, but both Tehran and the Houthis deny it has provided weapons to them.
A five-day humanitarian cease-fire began Tuesday night, just hours after Saudi-led warplanes targeted the Shiite rebels and their allies.
Fox News’ Lucas Tomlinson and The Associated Press contributed to this report.