DUBAI, United Arab Emirates (AP) — The Iran war risked reigniting after the U.S. tried to force open the Strait of Hormuz for commercial shipping, though a ceasefire seemed to be holding Tuesday even after the United Arab Emirates said Iran fired missiles and drones at it.

Iran's powerful parliamentary speaker and chief negotiator, Mohammad Bagher Qalibaf, accused the U.S. of undermining regional security with the effort to end Iran's stranglehold on the strait and warned that Tehran will respond.

WATCH: Hegseth and Caine hold briefing a day after new U.S. effort in Strait of Hormuz begins

The U.S. military said two American-flagged merchant ships successfully transited the strait on Monday, the first day of the effort, and that it fired on Iranian forces, sinking six small boats that were targeting vessels.

Disputing Washington's claim of sinking six boats, an Iranian military commander said two small civilian cargo boats were hit on Monday, killing five civilians, Iran's state TV reported.

Ship tracking data showed a Panamanian-flagged crude oil tanker heading toward the center of the strait Tuesday morning after leaving an anchorage in the Persian Gulf, though it was unclear if it would try to pass through. The tanker had a stated destination of Singapore, according to the MarineTraffic ship tracking site.

Iran's effective closure of the strait, through which about a fifth of the world's trade in oil and natural gas typically passes, along with fertilizer and other petroleum-derived products, has sent fuel prices skyrocketing, rattled the global economy and proved a major strategic advantage in negotiations to end the war. Breaking that grip would deny Tehran a major source of leverage.

WATCH: U.S. and Iran truce tested over the Strait of Hormuz, threatening to reignite conflict

But such efforts risk reigniting the full-scale fighting that erupted when the U.S. and Israel attacked Iran on Feb. 28, prompting it to close the strait.

Iran accuses U.S. of ceasefire violation

Iran has called the new U.S. effort a violation of the fragile ceasefire that has held for more than three weeks.

In a post on X on Tuesday, Qalibaf accused Washington of undermining shipping security in the Strait of Hormuz, and warned that a "new equation" there is taking shape.

He signaled that Iran has yet to fully respond to the U.S. attempt to reopen the waterway, saying: "We know full well that the continuation of the status quo is intolerable for America; while we have not even begun yet."

His statement did not mention negotiations with the U.S. that are now in the form of passing messages via Pakistan.

Trump vows to reopen the strait

U.S. President Donald Trump on Sunday warned that Iranian efforts to halt passage through the strait "will, unfortunately, have to be dealt with forcefully."

He said the U.S. effort, "Project Freedom," was intended to aid tens of thousands of stranded seafarers on hundreds of ships stuck in the Persian Gulf since the war began.

WATCH: Analyzing the U.S. effort to reopen the Strait of Hormuz amid tensions

The U.S.-led Joint Maritime Information Center advised ships on Monday to cross the strait in Oman's waters, saying it had set up an "enhanced security area."

But shippers remained wary.

The UAE bore the brunt of Iran's retaliation

The United Arab Emirates' Defense Ministry said its air defenses had engaged 15 missiles and four drones fired by Iran. Authorities in the eastern emirate of Fujairah said one drone sparked a fire at a key oil facility, wounding three Indian nationals. The British military reported two cargo vessels ablaze off the UAE.

Indian Prime Minister Narendra Modi on Tuesday condemned the attacks, calling the targeting of civilians and infrastructure "unacceptable." On X, Modi said India stands in "firm solidarity" with the UAE, and stressed the need for safe and uninterrupted shipping through the Strait of Hormuz.

Tehran did not confirm or deny the attacks but Iran's Foreign Minister Abbas Araghchi early Tuesday said on X that both the U.S. and the UAE "should be wary of being dragged back into quagmire."

Pakistan and Saudi Arabia condemned the strikes against the UAE. The Saudi condemnation came despite increasingly strained relations between it and the UAE.

Strait of Hormuz closure has far-reaching consequences

The disruption of the waterway has squeezed countries in Europe and Asia that depend on Persian Gulf oil and gas, raising prices far beyond the region.

The U.S. meanwhile has enforced a naval blockade on Iranian ports since April 13, telling at least 49 commercial ships to turn back, according to its Central Command. It also has warned shipping companies they could face sanctions if they pay Iran for transit of the strait.

The blockade has deprived Tehran of oil revenue it needs to shore up its ailing economy. U.S. officials have expressed hope the blockade will force Iran to make concessions in talks on its disputed nuclear program and other longstanding issues.

Negotiations make little progress

Iran's latest proposal for ending the war calls for the U.S. to lift sanctions, end the blockade, withdraw forces from the region and cease all hostilities including Israel's operations in Lebanon, according to the semiofficial Nour News and Tasnim agencies, which have close ties to Iran's security apparatus.

Iranian officials over the weekend said they were reviewing the U.S. response. Tehran has claimed its proposal does not include its nuclear program and enriched uranium, long a driving force in tensions with the U.S. and Israel.

Iran wants other issues resolved within 30 days and aims to end the war rather than extend the ceasefire. Trump expressed doubt over the weekend that the proposal would lead to a deal.

Becatoros reported from Athens, Greece. Sally Abou AlJoud in Beirut, Lebanon and Sheikh Saaliq in New Delhi contributed to this report.