The U.S. military announced Thursday that it is conducting the 13th night of strikes against Iran as clashes escalate over shipping routes. Earlier, 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.

U.S. Central Command said the latest attacks are 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.

A short time later, Iranian state media reported explosions along the strait in Bandar Abbas and Qeshm, as well as to the northwest near Andimeshk and Omidiyeh.

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. The threat came as 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.

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.

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