Dubai. The US military said Monday that two American-flagged merchant ships had successfully transited the Strait of Hormuz and that Navy guided-missile destroyers in the Persian Gulf were helping to restore shipping traffic. It separately denied Iran’s claims to have struck an American Navy vessel.

The announcement came a day after US President Donald Trump announced a new initiative to help guide ships through the critical waterway for global energy. Iran has effectively closed the strait since the United States and Israel started the war on Feb. 28, rattling the global economy.

The US-led Joint Maritime Information Center has advised ships to cross the strait in Oman’s waters, saying it set up an “enhanced security area.” US Central Command did not say when the Navy ships arrived or when the merchant vessels departed.

It was unclear whether shipping companies and their insurers will feel comfortable taking the risk, given that Iran has fired on ships in the waterway and vowed to keep doing so. Hundreds of ships have been bottled up in the Persian Gulf for weeks.

Iran has said the new US effort is a violation of the fragile ceasefire reached in early April. Its control of the strait has allowed it to inflict pain on the global economy despite being outgunned on the battlefield.

The United Arab Emirates, meanwhile, issued an emergency missile alert urging residents to immediately seek shelter. It was the first such alert since the ceasefire went into effect in early April. There were no immediate reports of casualties or damage, and an all-clear order was later issued.

The US military has said the new initiative might involve guided-missile destroyers, more than 100 aircraft, and 15,000 service members, but has not specified what kind of assistance it would provide.

Trump's announcement that the US would “guide” ships out of the strait warned that Iranian efforts to block them “will, unfortunately, have to be dealt with forcefully.”

He described “Project Freedom” in humanitarian terms, designed to aid stranded seafarers, many on oil tankers or cargo ships, who have been stuck in the Persian Gulf since the war began. Crews have described to The Associated Press seeing drones and missiles explode over the waters as their vessels run low on drinking water, food, and other supplies.

Iran’s state-run IRNA news agency called “Project Freedom” part of Trump's “delirium.”

Iran’s military command on Monday said ships passing must coordinate with them.

“We warn that any foreign military force -- especially the aggressive US military -- that intends to approach or enter the Strait of Hormuz will be targeted,” Maj. Gen. Pilot Ali Abdollahi told state broadcaster IRIB.

The UAE on Monday accused Iran of targeting a tanker linked to its main oil company with two drones as it passed through the strait. It did not say when the attack took place. No injuries were reported.

The Joint Maritime Information Center urged mariners to coordinate closely with Omani authorities “due to anticipated high traffic volume.” It warned that passing close to usual routes, known as the traffic separation scheme, “should be considered extremely hazardous due to the presence of mines that have not been fully surveyed and mitigated.”

But the head of security for the Baltic and International Maritime Council, a leading shipping trade group, said no formal guidance or details about the US effort had been issued to the industry. Jakob Larsen questioned whether the effort was sustainable in the long run or envisioned as a more limited operation, and said there is a “risk of hostilities breaking out again” if it goes ahead.