After more than a year of bloody repression in Syria – and stalemate at the United Nations – Russia's apparent commitment to stop sending arms to the murderous regime in Damascus looks like real progress at last.

There is, of course, an immediate, practical effect. With up to $4bn-worth of outstanding military contracts with Moscow, President Bashar al-Assad cannot fail to feel the impact if the comments from a senior Russian official prove accurate. Even more important, however, is the reading of the diplomatic runes. And, in this regard, it would be as well not to celebrate too soon.

Moscow's support for the Assad regime runs far deeper than arms sales, even with the quid pro quo of its naval base at the Syrian port of Tartus taken into account. Military deals are the fruit of the relationship between the two countries, not its substance, and it will take more than a new-minted arms embargo to signal that Russia is about to drop one of its longest-standing allies in the Middle East.

There are two forces at work here. One is the Russian leadership's strong desire to avoid being bounced by the UN into another military intervention. Libya is far from forgotten, as Vladimir Putin's tart references to "unilateral moves that are contrary to international law" this week attest. It would also be a mistake to overlook the strong historic bonds between Russia and Syria, which stretch back to Mr Assad's father, Hafez, at the height of the Cold War. Such ties – which include both trading relationships and significant migrant populations – are not easily unravelled.

That said, calling a halt to arms sales, even only to new shipments, has considerable symbolic value. It is the strongest reminder so far that there are limits even to Russia's support and, as such, it materially increases the pressure on Mr Assad. Over time, such things do count.

Moscow is not yet off the hook. Its support remains a vital prop for a regime that has proved itself wholly barbaric. Refusing to provide Damascus with weapons and warplanes to use against its own people is the least Russia could do. But it is welcome, nonetheless.