The widening military conflict in the Persian Gulf quickly began to disrupt shipping in one of the world’s biggest oil-and-gas producing regions, threatening to send energy prices soaring.

Oil markets were closed on Saturday when the United States and Israel launched attacks on Iran that spurred retaliatory strikes across the Middle East.

But shipping companies quickly began trying to steer clear of the region and the Strait of Hormuz, a narrow waterway on Iran’s southern border through which ships typically carry about one-fifth of the world’s oil and a significant amount of natural gas to market.

The Iranian military cautioned vessels to avoid the strait on Saturday, saying that “passage through the strait is currently unsafe,” according to Tasnim, a news outlet affiliated with the Islamic Revolutionary Guard Corps. But a U.S. official said on Saturday that there was no evidence Iran was attempting a military blockade of the strait. And though traffic through the strait slowed considerably on Saturday, ships were continuing to traverse the waterway, according to S&P Global Energy’s Commodities at Sea.

Any sharp and prolonged reduction of tanker traffic through the strait, even a voluntary curtailment by shipping companies, could significantly cut the flow of oil from big producers, including Saudi Arabia, Iraq and Iran, and push up prices.

“This will be a shock to the oil market,” said Daniel Yergin, a Pulitzer Prize-winning energy historian and vice chairman of the research firm S&P Global.

The U.S. Department of Transportation on Saturday warned commercial shipping to stay away. “It is recommended that vessels keep clear of this area if possible,” the agency said in a statement.

“Nobody’s going to enter right now,” Angeliki Frangou, the chief executive of Navios Maritime Partners, a Greek shipping company with vessels in the region, said, referring to the strait.

Countries such as Saudi Arabia and the United Arab Emirates have other ways of exporting some of their oil, but other major producers in the region, including Iraq and Qatar, do not have such options, according to the Center for Strategic and International Studies.