The key members of the OPEC+ oil alliance announced a greater-than-expected increase to production quotas on Sunday following United States and Israeli strikes on Iran that triggered retaliation by Tehran across the Middle East.

The "Voluntary Eight" group in the alliance, which includes top oil producers Saudi Arabia and Russia, said they had agreed a "production adjustment" of 206,000 barrels per day.

"This adjustment will be implemented in April," they said in a statement.

The text did not mention the outbreak of the Iran conflict, instead citing "a steady global economic outlook and current healthy market fundamentals" as reasons for the increase.

Before the weekend's meeting, experts had forecast a more modest increase of 137,000 barrels per day.

However, Jorge Leon, an analyst at Rystad Energy, warned the agreed increase was potentially not large enough to prevent the Iran conflict causing a spike in oil prices when trading opens on Monday.

Leon pointed to the possibility that Iran could target the Strait of Hormuz — a key waterway through which around nearly one-fourth of the world's seaborne oil supplies — in retaliation.

Iran's Revolutionary Guards have contacted ships to announce the strait is closed. On Sunday, Iranian state TV said an oil tanker in the strait was struck while attempting to "illegally" pass through and was sinking, showing footage of a burning tanker at sea.

"If oil cannot move through Hormuz, an extra 206,000 barrels per day does very little to ease the market," Leon said, arguing that "logistics and transit risk matter more than production targets right now".

The OPEC+ move "is unlikely to calm markets", he said.

"Prices will respond to developments in the Gulf and the status of shipping flows, not to a relatively small increase in output."

Besides Russia and Saudi Arabia, the Voluntary Eight group within OPEC+ includes Kuwait, Oman, Iraq and the United Arab Emirates, all of which were targeted by Iranian attacks for a second day on Sunday.

Algeria and Kazakhstan are also part of the group.

Another analyst, Stephen Innes, managing partner at SPI Asset Management, said that, with the fear of incoming missiles in the Strait of Hormuz, insurers canceling contracts for vessels wanting to go through there, and jammed electronic signaling in the Gulf region, commercial shippers were scared.

They are "starting to act as if the route is compromised", he said.

"A full closure for more than a few days is a nightmare scenario," he said.

A blockage of the strait could mean oil prices leaping from around $72 before the conflict to $120 to $150 a barrel when trading starts on Monday, he said, based on industry estimates.

He and other analysts pointed to land pipelines Saudi Arabia and the UAE could use to get around shipping through the strait, but noted that would still leave a shortfall of some 8 million to 10 million barrels per day on the market.

"Those are meaningful pressure valves, but they are not a replacement for the full seaborne flow," Innes said.

cuihaipei@chinadaily.com.cn

Agencies contributed to this story.