Another showdown on the high seas? 

White House and Pentagon officials said Tuesday the U.S. is monitoring an Iranian-flagged cargo ship headed in the direction of Yemen -- coming just days after the U.S. tracked a suspected Iranian arms shipment in the region. 

This time, the ship could be carrying little more than relief supplies to Yemeni citizens. 

But Pentagon officials could not say Tuesday whether the ship might have weapons onboard. And both Pentagon and White House officials suggested that even if the ship is carrying only aid, such a shipment would be going outside the established United Nations process. 

White House Press Secretary Josh Earnest called such a move "provocative" on Iran's part. 

"We're monitoring the shipment," Earnest said, urging Iran to go through the U.N. to deliver any aid. 

"Iran understands that they can't afford to play games with humanitarian assistance to people who are in dire need," Earnest said. "The Iranians know as well as anyone that a political stunt to defy their regional rivals outside the U.N. system is provocative and risks a collapse of the U.N.-led humanitarian cease-fire." 

Earnest was referring to a cease-fire in Saudi Arabia's air campaign against the Iran-backed Houthi rebels in Yemen set to begin Tuesday. 

Pentagon spokesman Col. Steve Warren also said the Iranians claim the ship is carrying humanitarian aid, and if this is the case, "then we certainly encourage the Iranians to deliver that humanitarian aid to the United Nations humanitarian aid distribution hub," located in Djibouti. 

He added: "If the Iranians are planning some sort of stunt in the region, then they know as well as we do that it would be unhelpful and potentially threaten the cease-fire." 

One official told Fox News the ship left Iran on Monday. The cargo ship is currently in the Gulf of Oman, proceeding in the direction of Yemen, according to the official. 

Another defense official told Fox News, "This cargo ship is not like the last convoy. It is an Iranian-flagged commercial vessel owned by Islamic Republic of Iran Shipping Line and does not have an escort at the moment." 

Last month, a nine-ship Iranian convoy was suspected of carrying weapons to Houthi rebels in Yemen. At the time, the U.S. Navy sent the aircraft carrier USS Theodore Roosevelt and a guided-missile cruiser to monitor -- and potentially intercept -- the convoy, which ended up turning around four days later and returning home. 

An Iranian news agency reported Tuesday that Iranian warships will eventually accompany the latest cargo ship heading toward Yemen. 

But the Pentagon stressed that, currently, the lone Iranian ship does not have a military escort. 

"At this point, there is no reason to believe the ship is anything except what the Iranians say it is -- a ship carrying supplies," said one official. 

When Fox News asked the official how he could be sure the Iranian cargo ship steaming toward Yemen is only carrying humanitarian supplies and not weapons, he pointed to the intelligence-gathering capability of the U.S. military. 

"We monitor everything.  We monitor the activities of all maritime traffic in the region including this one," he said. 

The Pentagon is not tracking any other ships headed in the direction of Yemen at this time besides the cargo ship, but they are aware of the press reports coming out of Iran from its state-run news agency. 

Fox News' Lucas Tomlinson contributed to this report.