Oil prices surged and stocks tumbled worldwide in early trading on Thursday as Iran escalated shipping attacks in a critical tanker route.

Global crude spiked above $100 a barrel on Thursday before settling slightly below that key benchmark. The rise in oil prices defied a U.S. effort hours earlier to reassure markets with an announcement of the second-largest ever release from the nation’s petroleum reserve.

A selloff hit Wall Street as traders feared economic fallout from a potentially prolonged bout of elevated oil prices.

The Dow Jones Industrial Average fell 550 points, or 1.1%, while the S&P 500 dropped 0.8%. The tech-heavy Nasdaq declined 0.8%.

Oil markets are suffering a major supply shortage due to an Iranian blockade of the Strait of Hormuz, a trading route that facilitates the transport of about one-fifth of the global oil supply.

U.S. gasoline prices jumped to $3.59 on Thursday from $2.94 a month earlier, AAA data showed.

Indexes fell worldwide on Thursday as the jump in oil prices rippled through global markets. Tokyo’s Nikkei 225 index dropped 1.2%, while pan-European STOXX 600 index slipped 0.5%.

Global crude oil prices hovered at about $99 per barrel on Thursday morning, which marked a 7.5% increase from a day earlier. Since a month ago, oil prices have soared 43%.

In recent days, President Donald Trump has voiced mixed messages about how the White House may address oil prices and related cost woes.

Trump has indicated the war may end soon, but he has also threatened to escalate the conflict. On Tuesday, he said on social media that Iran would “be hit by the United States of America TWENTY TIMES HARDER” if it sought to impede tanker traffic in the Strait of Hormuz.

This is a developing story. Please check back for updates.