Somali pirates hijacked a suspected Iranian shadow fleet tanker off the coast of Yemen on Thursday, amid what experts say is the biggest surge in piracy in the region in more than a decade.

The oil tanker, the SIBU 1, flies an Eritrean flag. But it is likely part of Iran’s shadow fleet of ships exporting oil around the world, according to TankerTrackers.com, a company that monitors global oil shipping. The tanker has routinely ferried fuel to an oil terminal in the Yemeni port of Ras Isa that is controlled by the Iranian-backed Houthi militia, the company added.

SIBU 1 was the second ship hijacked by Somali pirates in a span of four days, according to officials in Somalia. Days earlier, a Cameroon-flagged cargo vessel carrying weapons was seized off the coast of the semiautonomous Somali region of Puntland, said Abshir Hashi Ali, the director general of the ministry of ports in the neighboring Galmadug State.

Years after a global naval campaign largely curbed the scourge of piracy, experts worry that the conflict in Iran — along with other factors, like regional rivalries and economic difficulties for local fishermen — is setting the stage for a comeback.

In the first five months of the year, more than 17 piracy-related incidents were documented across Somali waters and the Gulf of Aden, including the hijacking of at least three commercial ships and five dhow boats, according to the Center for Maritime Security.

Mr. Hashi, the local ports official, said that six ships are currently being held by Somali pirates, including the two seized this week. Across those ships, he added, more than 100 crew members are being held hostage.

The resurgence of piracy comes at a time when maritime blockades and fighting in crucial shipping routes — including the Red Sea and Strait of Hormuz — have widened the opportunity for hijackings and ransoms.

“The most frequent successful spate of piracy attacks in almost a decade and a half are going virtually unnoticed on account of just how bad the maritime security picture is in the wider region,” said Ian Ralby, the president of the maritime governance nonprofit group, Auxilium Worldwide.

Col. Mohamed Jama, a senior security official at the Puntland Maritime Police force, said the SIBU 1 had been boarded by seven armed pirates about 130 miles off the coast of Yemen and diverted to Somali waters. He said that the condition of the 20-person crew was unknown.

The Cameroon-flagged cargo ship seized on Monday, the Lutuf, was sailing from Turkey to the Somali capital, Mogadishu, said Mr. Hashi, the local ports official.

Turkey and other regional powers, like the United Arab Emirates, have been vying for influence in Somalia. While Turkey has aligned with the government in Mogadishu, where it has a large military facility and trains dozens of Somali forces, the Emiratis have deepened their ties with the semiautonomous Puntland region, strategically located where the Gulf of Aden meets the Indian Ocean.

Hours after the Lutuf was hijacked off Puntland’s coast airstrikes targeted a suspected Somali pirate boat that had interacted with the pirates aboard the hijacked ship, Mr. Hashi said.

At least four people were killed in the strike, whose origin was unclear, he said.

Somali piracy has been a major problem in the Red Sea in recent decades, reaching a peak around 2010 when hundreds of vessels were seized, costing shipping companies and insurers billions of dollars, in part through ransom fees.

The numbers began to decline in part because of international naval coalitions involving NATO and the European Union. Ships also began traveling farther from Somalia’s coast and adding deterrents like water cannons.

In recent years, missiles and drones fired by the Houthis at Red Sea shipping in support of Hamas’s war against Israel in Gaza have posed a bigger threat to commercial vessels than piracy. The Houthis, a Yemeni militant group backed by Iran, paused their attacks on vessels last year. But they resumed threatening shipping in the sea this summer after declaring a blockade on Saudi Arabia in the Red Sea.

A series of attacks near Yemen’s coast has raised suspicions that Somali pirates and the Houthis are collaborating, experts and local officials say.

In May, Somali pirates hijacked an oil tanker off the coast of Yemen, the Togo-flagged Eureka, and directed it through the Yemeni part of the Gulf of Aden toward Somalia’s coast.

The Houthis and Somali pirates have cultivated an alliance over the years, with the militia at times providing technology and military backup. Analysts say the war in Iran and the resulting disruptions to oil shipping could give them more reason to work together as the high price of fuel presents an opportunity to profit.

But experts say there are also longer-term reasons for the current rise in piracy, like increased competition from Chinese and Spanish trawlers that has made fishing less lucrative in coastal communities, and the decline in maritime policing that armed gangs have taken advantage of.

Declan Walsh contributed reporting.