The British navy-affiliated agency UKMTO said it had a report of a projectile hitting a vessel as it moved through the Strait of Hormuz.
The amount of damage and the status of the crew were unknown as of early Sunday, the agency said.
UKMTO's report came the day after Saudi Arabia said it had temporarily closed its East-West pipeline as a precaution after a drone attack that both Baghdad and Riyadh said had originated in Iraq, where Iranian-backed militias operate.
Ongoing attacks in the widening Middle East war could send energy prices even higher after a week that saw the cost of Brent crude surge back above $US100 ($A139) a barrel.
The 1200km East-West pipeline running across the Arabian Peninsula has served as the main route out for global supplies of Middle East oil for the past six months while the Strait of Hormuz has been largely shut by war.
It has been moving four-to-five million barrels per day, amounting to four-to-five per cent of global supply, sparing Saudi Arabia the brunt of the disruption that has crippled other Gulf oil and gas exporters.
There was no immediate claim of responsibility for the strikes on the pipeline.
Satellite images showed black smoke rising from an area of the pipeline south of Medina.
The attack resulted in some injuries and damage that was being assessed, the Saudi Foreign Ministry said without detailing the impact on exports.
US President Donald Trump, visiting Ireland to attend a golf tournament at one of his resorts, said Iran was probably to blame.
On Saturday, the Saudi Civil Defence said in a statement that a projectile fired by the Houthis had injured two people in Jazan region, and damaged several buildings, including a mosque.
Washington now faces a dilemma over whether to help the Saudis fight the Houthis, risking further expansion of the war.
Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman, phoned Trump on Thursday and asked for US military help fighting the Houthis, three sources told Reuters.
They said the prince was told that Washington would not intervene directly for now but would help with intelligence.
Trump confirmed on Saturday that he had spoken to the crown prince, and said the Houthis had also phoned his administration and asked Washington not to become directly involved in the conflict.
Four Yemeni government sources told Reuters that the Houthis had seized the strategic island of Perim on Friday in the middle of the Bab el-Mandeb strait - the "Gate of Tears" at the mouth of the Red Sea.
