CAIRO (AP) — The U.S. military announced the 13th night of strikes Thursday night against Iran as clashes escalate over shipping routes. The attacks came after Yemen’s Iran-backed Houthi rebels said they attacked two Saudi oil tankers in the Red Sea, potentially widening the Iran war as international oil topped $100 a barrel.

A short time later, Iranian state media reported explosions along the strait in Qeshm and Bandar Abbas, where two people were reportedly injured, as well as to the northwest near Andimeshk, Omidiyeh and Firuzabad.

U.S. Central Command said the latest attacks concluded shortly before 5 a.m. local time. The strikes were designed to “further degrade Iran’s ability to threaten civilian mariners and commercial vessels transiting regional waters” as the Americans push to regain control over the Strait of Hormuz and restore the flow of international shipping.

Meanwhile, the Houthis threatened to shut down another key trade route, with the world economy already reeling from Iran’s closure of the Strait of Hormuz. Iran and the U.S. have stepped up attacks as they vie for control of the strait, through which a fifth of the world’s oil and gas transited in peacetime, setting off a scramble for alternative routes.

President Donald Trump threatened “major military punishment” against the Houthis if their attacks on ships continue.

“If they do this again, the U.S. will hold Iran responsible, in that the Houthis are a Surrogate and/or Proxy of Iran, and major military punishment will be inflicted upon Iran and, of course, the Houthis, themselves,” Trump wrote on social media.

As the rhetoric escalated, so did the economic fallout. The price of Brent crude oil, the international standard, spiked more than 6% Thursday, to about $100 a barrel. That is the highest level since May, before the two sides reached a preliminary peace agreement last month that has since collapsed.

Prime Minister Ali al-Zaidi of Iraq, which has close ties to both the U.S. and Iran, was in Tehran on Thursday to call for peace and dialogue, and he promised not to allow Iraqi territory to be used for attacks on Iran, his office said. Al-Zaidi met with Trump earlier this month in Washington.

U.N. Secretary-General Antonio Guterres warned Thursday that the region is being pulled “into an ever-widening circle of confrontation” with one crisis feeding another and every escalation triggering the next.

“Diplomacy is the only way forward,” Guterres told the U.N. Security Council. He called for Pakistan’s efforts to mediate an end to the war to be strengthened.

The Houthis’ SABA news agency said the rebels had struck two tankers, the Encelia and the Layla, in the Red Sea, causing fires on both. There were no reports of casualties.

It was the first reported Houthi attack on a vessel since they announced a blockade of Saudi-linked shipping through the Bab el-Mandeb Strait earlier this week in retaliation for the kingdom’s blockade on Yemen and a recent attack on the international airport in Yemen’s rebel-held capital, Sanaa.

Bab el-Mandeb, at the southern tip of the Arabian Peninsula, is a vital shipping chokepoint connecting the Red Sea to the Gulf of Aden. Around 12% of the world’s trade, including a fourth of global container traffic, passes through there, moving between Europe and Asia via Egypt’s Suez Canal.

The state-run Saudi Press Agency reported an attack set the Encelia ablaze overnight in the Red Sea. It did not mention the Layla.

The United Kingdom Maritime Trade Operations Center said it received a report of a tanker being struck by “an unknown projectile” 80 miles southwest of Al Shuqaiq, Saudi Arabia, in the Red Sea.

The Houthi attacks put at risk oil shipments from Saudi Arabia’s Yanbu port and present a “double whammy” on top of the disruption in the Strait of Hormuz, said maritime data firm Lloyd’s List Intelligence.

Saudi Arabia has diverted millions of barrels a day of oil exports to Yanbu via an overland pipeline as the war has bottled up the Persian Gulf.