The MV Alaed had its insurance withdrawn by The Standard Club in London while it was about 50 miles (80.4km) off Scotland's north coast.
Withdrawal of insurance prevents the MV Alaed from sailing until its owner could secure new cover.
UK Foreign Secretary William Hague said the ship was returning to Russia.
It was thought that the vessel had stopped for a time off the Western Isles.
The Russian embassy in London has not yet commented.
The UK and US have raised concerns with Russia about shipments of weapons to Syria, which is subject to a European Union arms embargo.
In a statement, the FCO 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 14 June that all defence 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."
The Standard Club said it believed the MV Alaed had been off the coast of the Western Isles when insurance was withdrawn.
It said the cover was withdrawn because the owners of the ship had "broken internal rules" of the club - a mutual insurance association - and not on the instructions of the UK government.
The club said it had sought more information on the boat's cargo.
In a statement, it said: "We were informed on Friday evening that the ship might be carrying weapons, in particular attack helicopters, missiles and non-specific munitions, and we are making inquiries to establish what their side of the story is.
"There are exclusion clauses in our cover and, for anyone involved in improper or unlawful trade, we can cancel cover.
"We are investigating whether or not to do so in this case."
In December last year, Russian warships and support vessels believed to have been headed for Syria took shelter in Scotland's Moray Firth during bad weather.
HMS York tracked aircraft carrier Admiral Kuznetsov, a destroyer, two frigates, an ocean-going tug and three oil tankers until the task group left and heading into the Atlantic.