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 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 Saturday, saying that “passage through the strait is currently unsafe,” according to Tasnim, a news outlet affiliated with the Iran’s Revolutionary Guard. But a U.S. official said Saturday that there was no evidence Iran was attempting a military blockade of the strait. And though traffic through the strait slowed considerably Saturday, ships were reportedly continuing to traverse the waterway.