Russia is to delay its controversial shipment of three attack helicopters and an air defence system to Syria until full security control is restored there, the Interfax news agency reported Friday.

The military cargo has already been moved into storage after being unloaded from a cargo vessel that tried to make the dispute delivery last month before being exposed by Washington, according to the news agency.

An unnamed Russian military official told the news agency that recent gains by the armed opposition and Wednesday’s deadly attack on President Bashar al-Assad’s inner security circle had forced Moscow to reassess the delivery.

“The decision to delay the Syrian delivery time frame is based on the escalating military and political situation in the country,” the unnamed military source told the news agency.

The source said other considerations included “the act of terror in which several senior Syrian officials were killed and injured as well as the escalation in military activities by the opposition.

“In these conditions, the authorities are not able to guarantee the safe reception of the helicopters,” the source said. There was no official confirmation of the report from the defence ministry.

Moscow has enraged the West by ignoring a US-EU arms embargo and extending military ties with Damascus that stretch back to the Soviet era. Those ties have continued despite 16 months of violence that the opposition says have claimed more than 17,000 lives.

Russia argues that the Mi-25 choppers already belonged to Syria and were only returned to Russia for upgrades under a 2008 contract signed long before the fighting began.

But US Secretary of State Hillary Clinton has dismissed that defence. She has repeatedly called on Russia to break all ties — including diplomatic ones — with Assad.

Signs that Russia was slowly starting to shift its military trade position emerged earlier this month. A top arms export official said Moscow would not supply any of its most advanced systems to Damascus until the situation calmed down.

The reports said Russia was now moving the helicopters into storage rather than boarding them immediately onto a plane or another ship in a bid to quickly transport them to Assad’s army.

A source at a Kaliningrad region port near Poland that the cargo vessel visited on Thursday told Interfax that the helicopters were returned to the same repair facility that had earlier upgraded the equipment.

“The equipment will remain in safe storage until other instructions are issued,” the unnamed source told the news agency.

The Alaed cargo vessel on Friday had reportedly approached a port in Russia’s second city of Saint Petersburg after leaving Kaliningrad.

Its private owners Femco said earlier the ship is already under contract to make an unidentified delivery to the Pacific coast of Russia’s Far East.