Russia likes to tell the world it is playing the adult in Syria – vetoing hasty UN security council resolutions, calling for reticence and political dialogue. But behind that diplomatic mask, the Kremlin is nursing its ally, shipping hundreds of millions of dollars worth of weapons to Assad's rampaging army and killing all efforts to end the violence. Syria imports almost all the weapons and munitions its huge army uses: Russia supplies nearly three-quarters of them.

Russia's state-owned weapons dealer, Rosoboronexport, continues to send shipments of heavy arms to Syria's military. On Tuesday, US secretary of state Hillary Clinton accused Moscow of preparing to send Assad another batch of the very attack helicopters now hovering over Syria's civilian neighbourhoods. Russia's deadly deceit must be stopped – and here's how we can do it.

Old friends stick together

Russia's alliance with Syria goes back decades. Under Bashar al-Assad's father, Hafez, the Soviet Union trained and armed the Syrian army – then an important check on US and Israeli power in the Levant. Since the collapse of the USSR, and the transfer of power from father to son, bilateral ties have not weakened. Syria hosts Russia's only Mediterranean port, and is a major customer for its arms.

Since 2006, Russia has sold Syria more than $4bn worth of weapons. Assad's purchases account for roughly 10% of Russia's weapons exports – last year alone, arms sales (including advanced missile systems) totalled nearly $1bn. Rosoboronexport, the state-owned weapons dealer, has sent two shipments of arms to Syria in the last two months alone.

What do those arms shipments translate to on the ground? More than 13,000 dead Syrians, 1,000 of them children. And now, in the wake of the Houla massacre, which left 49 children dead, and another rampage in Hama that killed 27 kids, it seems Russia is preparing to send Assad more attack helicopters.

It's not personal, it's business

Syria is an important Russian ally, but it isn't Rosoboronexport's biggest buyer. That honour belongs to India (the US is another key client) – and these facts offer the global community a golden opportunity to act. The Russian leadership's unwillingness to abandon Syria is clear, but if India and the US cancelled weapons deals, that would cost a lot of money – and the threat will force Russia's leaders to re-evaluate their position.

There are promising signs that the US and India do not want to be tied to a state-owned exporter dealing death in Syria. The US has awarded a no-bid contract worth up to $1bn to the company (which it had blacklisted until 2010) to supply the Afghan army with Mi-17 helicopters and spare parts. But now 17 US senators have written to the defence department asking why the US is paying the same company funnelling arms to Syria. The US has already persuaded the company to stop selling small arms to Assad's regime.

A show of arms in Paris

This week, the French defence ministry is hosting an international arms fair, called Eurosatory, just outside Paris. There are 1,300 exhibitors from 70 countries – and Rosoboronexport is one of them.

On Tuesday, Avaaz organised an event in Paris. The participants included local Syrian activists, a 14-year-old boy who escaped from Syria last week, and leading opposition figure Dr Kamal al-Labwani. In front of a sign that read "Russian arms, killing Syrian kids" was a pile of toys for the 1,000 children who have died in the violence. Russia's arms dealer has brought death to Syrians, so we figured we should help bring Syrians forward to confront them.

Speaking at the event, al-Labwani said: "We tried the the Jasmine revolution route, but it failed. The international community has failed to stop the violence. Now we're left with a whole series of grievous options. But stopping Russia's support for the regime could make a real difference."

Take action: Join our call. Sign the petition asking Indian prime minister Manmohan Singh and US president Barack Obama to cancel existing and future contracts with Rosoboronexport.