What to know as the US tries to open the Strait of Hormuz and a ceasefire wavers

Few ships had appeared to take advantage of “Project Freedom,” announced Sunday by President Donald Trump, though the U.S. said two U.S.-flagged merchant ships safely transited with its help.

Caution, even skepticism, is growing among shippers, and markets, over the lack of details from Washington. Who would risk their crew and cargo to possible Iranian fire?

Here’s what to know.

U.S. appears to be going it alone

Iran’s grip on the crucial waterway has left hundreds of commercial ships and tens of thousands of sailors stuck since the war broke out over two months ago. The U.S. military says 87 countries are represented among the vessels.

Backed up in the strait are weeks’ worth of supplies of globally needed oil, gas, fertilizer and other goods. This has been Iran’s strategic advantage in the war, one that has pinched economies and dimmed the outlook for the Republican president's party in this year’s midterm U.S. elections.

Oil prices rose Monday as uncertainty continued around the strait and the U.S. effort, which Trump has described as a humanitarian one to help countries that have been “neutral and innocent” in the war.

While countries in Europe and elsewhere have fretted over the strait and have been urged by Trump to help solve the issue, it was not immediately clear whether any other nation was involved Monday.

The U.S. military, which has been blockading Iranian ports for weeks, said the initiative involves guided-missile destroyers, more than 100 aircraft and 15,000 service members but did not say how they are deployed.

Trump warned that interference in the effort “will, unfortunately, have to be dealt with forcefully.”

The U.S. military on Monday said it sank six small Iranian boats that were targeting civilian vessels, and said Iran launched missiles and drones at ships the U.S. was protecting.

Adm. Brad Cooper, who heads U.S. Central Command, said Iran initiated the "aggressive behavior." He declined to say whether the ceasefire was over.

Iran calls the effort part of Trump’s ‘delirium’

Iran effectively closed the strait by attacking some ships over the past two months, and told others not affiliated with the U.S. or Israel that they could pass if they paid a toll.

Iran’s military command on Monday said ships still must coordinate with Tehran to transit the strait and warned that “any foreign military force — especially the aggressive U.S. military — that intends to approach or enter the Strait of Hormuz will be targeted,” the state broadcaster reported.

Iranian news agencies claimed that Iran struck a U.S. vessel southeast of the strait, accusing it of violating "maritime security and navigation norms.” The U.S. military denied it.

Concerns remain about Iranian mines in the waterway. Cooper said the U.S. military had cleared a pathway in the strait, and set up a “defensive umbrella” that includes helicopters and fighter planes to protect freighters leaving it.

Iran’s state-run IRNA news agency has called Trump’s plan to reopen the strait part of his “delirium.”

Wary shippers say security situation is unchanged

The threat level around the strait remains critical, according to the U.S.-led Joint Maritime Information Center, even as it issued an advisory on the new U.S. effort.

The head of security for the Baltic and International Maritime Council, a leading shipping trade group, said no formal guidance or details about the effort had been issued to the industry.

Without Iran's consent for safe transit, "it is currently not clear whether the Iranian threat to ships can be degraded or suppressed,” Jakob Larsen, BIMCO’s chief safety and security officer said in a statement.

Larsen questioned whether the U.S. effort is sustainable in the long run or is envisioned as a limited operation, and said there is a “risk of hostilities breaking out again” if it goes ahead.

“There’s not much clarity at this point,” United Nations spokesperson Stephane Dujarric told journalists.

The Joint Maritime Information Center has advised ships to cross the strait in Oman’s waters, saying the U.S. has set up an “enhanced security area.” It warned that passing close to usual routes “should be considered extremely hazardous due to the presence of mines that have not been fully surveyed and mitigated.”

The center urged mariners to coordinate closely with Omani authorities “due to anticipated high traffic volume" — a forecast that on Monday seemed unlikely to play out.

Adam Schreck in Dubai, United Arab Emirates, Fatma Khaled in Cairo, and Edith M. Lederer at the United Nations, contributed to this report.