LONDON — A British maritime insurer revoked its coverage of a Russian cargo ship on Tuesday because it was carrying refurbished Russian-made attack helicopters to Syria, potentially stalling the delivery.

The ship, the 400-foot MV Alaed, owned by the Russian shipping company FEMCO , was last tracked about 100 miles to the west of Scotland early Tuesday, according to data available online . The state-owned Russian news agency Ria Novosti reported that it was carrying “a cargo of Mil Mi-25 attack helicopters” and “coastal-based anti-ship missiles” to Syria.
Its insurer, the Standard Club, said in a statement that the coverage was withdrawn, raising the prospect that the ship would be delayed as it sought an alternative, because its cargo had breached Standard Club's rules. "We were made aware of the allegations that the Alaed was carrying munitions destined for Syria," the statement said. "We have already informed the shipowner that their insurance cover ceased automatically in view of the nature of the voyage."
It was not immediately clear why the cargo ship was off the coast of Scotland on its voyage to Syria or whether the ship would continue in defiance of the insurance policy withdrawal. Calls to the FEMCO management office went unanswered.
The insurer denied reports in the British news media that it had come under any government pressure, and a spokesman for Britain's Foreign Office declined to comment, saying only that officials were "aware of a ship carrying a consignment of refurbished, Russian-made attack helicopters heading to Syria."
The statement said Foreign Secretary William Hague had expressed Britain's view to his Russian counterpart, Foreign Minister Sergey V. Lavrov, when they met on June 14 that "all defense shipments to Syria must stop. We are working closely with international partners to ensure that we are doing all we can to stop the Syrian regime's ability to slaughter civilians being reinforced through assistance from other countries."
Russia is the most important arms supplier to the government of Syria President Bashar al-Assad and has faced increasing accusations, led by the United States, that Russian weapons are abetting his harsh suppression of a 16-month-old uprising. Last week Secretary of State Hillary Rodham Clinton accused the Kremlin of supplying attack helicopters to Mr. Assad. Russia said all of its military shipments to Syria are defensive in nature.
Rick Gladstone contributed reporting from New York.