A Russian ship carrying attack helicopters to the Syrian government turned back toward home Tuesday after its London-based insurance carrier canceled coverage, following a warning from the British government that the delivery would violate European Union sanctions.

U.S. and British officials said the ship, which had reached the North Sea near Scotland, appeared to be returning to its home port on the Baltic Sea.

The Obama administration declined to claim credit for stopping the journey of the Russian-owned merchant vessel Alaed. But the insurance tactic was originated by the Treasury Department four months ago to prevent at least two shipments of Syrian oil from reaching the open market via Iran, a senior administration official said.

In the earlier episodes, the American insurer of a Liberian-flagged vessel and the European insurer of a Maltese-flagged ship both canceled coverage after being told they were violating U.S. and E.U. sanctions prohibiting any involvement in arms shipments or economic support for Syria, according to the official, who spoke on the condition of anonymity about the sensitive issue.

The administration shared the tactic with its European allies, the official said. E.U. sanctions implemented two months ago prohibit insurance on Syrian goods.

About a dozen M-25 helicopters aboard the Alaed were first sold to Syria during the Soviet era and had been returned to Russia some time ago for refurbishment. The Obama administration charged last week that the aircraft, which it said were on their way back to Syria, would be used in President Bashar al-Assad’s attacks against civilians.

The charge came amid an increasingly tense diplomatic standoff between Moscow and Washington. Secretary of State Hillary Rodham Clinton said last week that Russia was supplying the Syrian military with the means to intensify its campaign against the rebel opposition. Russia, one of Syria’s strongest allies, has refused to join the U.S.-led campaign to pressure Assad to honor a U.N.-brokered cease-fire and negotiate a transfer of power.

President Obama appeared to make little headway at the G-20 summit in Mexico in persuading President Vladi­mir Putin to join the international community in insisting that Assad step aside. Moscow has charged that the United States and its allies are fueling the Syrian conflict by assisting Syrian rebels.

During a closed-door briefing at the United Nations on Tuesday, senior U.N. officials provided the Security Council with a grim account of their observer mission in Syria. Peacekeeping chief Herve Ladsous said he will probably have to reconfigure the mission because of the risks to observers, who have been repeatedly targeted by hostile crowds and come under indirect fire, and the lack of commitment by all parties to a political transition, according to a diplomat in the room.

Syria’s ambassador to the United Nations, Bashar Al-Jaafari, told reporters outside the Security Council meeting that “we are a state, and we are free to buy weapons from wherever we want . . . there is no violation of international law.”

Ladsous and mission chief Gen. Robert Mood told reporters that, while the mission remains suspended, they have no immediate plans to shut it down.

In a radio interview Tuesday, Republican presidential candidate Mitt Romney repeated his charge that Obama has showed weakness and “extraordinary naivete” toward Russia. He asserted that Russia is a “geopolitical foe” that “consistently opposes our actions at the United Nations.”

Administration officials appeared pleased with the success of the insurance gambit on two levels: It stopped the helicopter shipment and averted a direct confrontation with Russia.

While calling the return of the helicopters to Syria “intolerable,” Pentagon spokesman Capt. John Kirby refused to gloat at a briefing for reporters. “I don’t think it suits anybody getting into any kind of finger-pointing game here at this point in time,” he said.

Kirby confirmed reports that Russia plans to send three naval vessels with supplies and personnel to the Syrian port of Tartus, where it has a major installation. But he played down the significance of the move, saying that the United States has “no indication that these vessels and that material” were for any purpose other than to “resupply and to help with force protection.”

The Russian Defense Ministry also quickly moved to deny reports that it is sending a warship to Syria or that it would conduct military exercises there along with Iran and China. Syria also denied such plans, which were reported by Fars, the Iranian news agency.

Speaking to Parliament on Tuesday, British Foreign Secretary William Hague first announced that the Alaed would not make its delivery, saying that “the ship that was reported to be carrying arms to Syria has turned back.”

The insurer, London-based Standard Club, said it canceled the coverage after being “made aware of the allegations that the Alaed was carrying munitions destined for Syria, which would be a clear breach of our rules.”

“We consequently informed the ship owner that the insurance coverage ceased automatically in view of the nature of the voyage,” a Standard Club spokesman said in a telephone interview.

“We would not support the breaking of E.U. sanctions,” said the spokesman, who refused to be identified or to comment on how the company was “made aware” of the sanctions issue. He said the Club, a mutual association of ship owners, had also canceled coverage for several other ships belonging to Femco, the Russian shipping company that owns the Alaed. “We’ve decided we don’t want that member in the Club anymore,” he said.

The Curacao-flagged, 400-foot Alaed is managed by a Danish company, United Nordic Shipping.

Correspondents Colum Lynch at the United Nations and Karla Adam in London contributed to this report.