The MV Alaed was halted close to the Outer Hebrides on Monday after the UK Government raised concerns it was carrying missiles and helicopters to the war-torn country.
The ship's insurer, the Standard Club, withdrew cover for the vessel after being alerted by the authorities, and Foreign Secretary William Hague yesterday confirmed it had turned back.
Under EU sanctions, countries are banned from exporting arms to Syria and providing other services for anyone who does.
Mr Hague told the House of Commons: "I am pleased the ship reported to be carrying arms to Syria has turned back, apparently towards Russia.
"We have in place a European Union arms embargo on Syria. We have had discussions with Russia about that specifically."
Russia has faced increasing Western criticism over arms supplies to Syria, where the United Nations says government forces have killed more than 10,000 people.
The development came as world leaders, including the US and Russian presidents, met at the G20 summit in Mexico.
President Barack Obama looked stony-faced as he sat beside President Vladimir Putin, a key ally of Syrian president Bashar al Assad, but said they were working together on a "political solution".
No mention was made of the ship, which according to ship tracking websites was at one point less than 55 miles off Port of Ness village in the Isle of Lewis.
The Foreign and Commonwealth Office (FCO) said it was monitoring the vessel, which moved out of EU waters after its alleged cargo was revealed.
A spokesman said: "We are aware of a ship carrying a consignment of refurbished Russian-made attack helicopters heading to Syria.
"The Foreign Secretary made clear to Russian Foreign Minister Lavrov when they met on June 14 that all defence shipments to Syria must stop.
"We are working closely with international partners to ensure we are doing all we can to stop the Syrian regime's ability to slaughter civilians being reinforced through assistance from other countries."
Insurer Standard Club said: "We were made aware of the allegations that the Alaed was carrying munitions destined for Syria. We have already informed the ship owner their insurance cover ceased automatically in view of the nature of the voyage."
The firm also withdrew insurance cover for seven other ships owned by the Russian shipping company Femco, which has so far refused to comment, in response to the incident.
Western Isles civic leaders said the incident showed the importance of having an emergency towing vessel on duty to protect the islands. Last week, the UK Government did a U-turn and agreed to reinstate a publicly funded emergency tug, but it has still to return.
Angus Campbell, leader of the Western Isles Council, said: "If the reports are true, that the vessel was sitting out at sea not far from the islands with a cargo of munitions as well as helicopters, it can only emphasise the threat from dangerous cargoes, the more so if it was no longer insured. If anything happened to a vessel like that, we have to have the capability of dealing with it very quickly. So the sooner the tug is back on station the better."
The Russian Embassy in London refused to comment.