Just a few days ago Russian warplanes, allegedly using cluster munitions, attacked a US-backed Syrian opposition group based near al-Tanf - a small town close to the border with Iraq.
The Syrian Observatory for Human rights has released images from the location that appear to show the tail-section of a typical RBK-500 cluster bomb canister. And the Americans - to put it mildly - are far from happy.
It is not just the use of the cluster munitions - which is controversial in itself. But it is also the target of the attack - a strike that the Russians have subsequently denied any responsibility for.
Immediately after the Russian air strikes on 16 June, the US invoked a high-level agreement to exchange views on the incident via a videoconference. For one thing the Americans were clearly enraged that a militia group they support was hit.
But the Pentagon insists that there were serious safety issues as well. US jets were diverted to the area after the initial strike. They tried to contact the Russian pilots and when they withdrew, more Russian jets appeared to carry out a second strike.
US and Russian warplanes operating in close proximity carries obvious dangers.
The Russians insist that the incident was caused by miscommunication on the part of the Americans. But to add an additional twist to the controversy, they have up to now shown little desire to prevent images of cluster munitions at their Syrian airfield from being distributed.
However Russia Today recently re-edited a news package dealing with the visit of the Russian defence minister to Syria, to remove the frames that showed RBK-500s on the pylons of a warplane.
But after a few hours they seemed to think better of it and - for whatever reason - re-instated the images.
There has been evidence of the consistent use of cluster munitions by Russian warplanes since the beginning of its air campaign in Syria.
Images of the RBK-500 family of weapons in particular have cropped up in several film reports carried by Russian television stations who have had reasonably unrestricted access to the base.
Human rights campaigners such as Human Rights Watch and Amnesty International have spoken out against the use of these weapons by the Russian air force in Syria since last year.
And despite Russian denials, open source intelligence monitors including the Bellingcat web site and the Russia-based Conflict Investigation Team have analysed the imagery, proving incontrovertibly that the weapons are in regular use on Russian aircraft operating out of Russia's base in Syria.
What are cluster munitions?
Cluster munitions are a controversial weapon for two main reasons. They are essentially a canister filled with smaller bomblets or sub-munitions. When the canister opens after being released from the aircraft, the sub-munitions are spread over a large area. Arms campaigners insist that they can thus not discriminate between military and civilian targets. Furthermore many of the sub-munitions typically fail to explode, littering the battlefield long after the fighting has moved on, posing a risk to civilians and particularly children.
That is why there have been long-standing efforts to ban them. A convention to this effect entered into force in August 2010 and to date some 119 countries have signed up, agreeing not just to destroy their stocks of these weapons but also to halt their manufacture.
But not all countries have signed up. In particular two of the major military players, Russia and the United States, have so far refused to sign. The US State Department's website declares that "cluster munitions have demonstrated military utility. Their elimination from US stockpiles would put the lives of its soldiers and those of its coalition partners at risk".
Indeed the US military claims that "cluster munitions can often result in much less collateral damage than unitary weapons, such as a larger bomb or larger artillery shell would cause, if used for the same mission". No doubt the Russians would take a similar view.
It is clear that this incident near al-Tanf has further soured relations between Washington and Moscow, underlining once again that the purpose of Russia's air strikes continues to be far broader than simply striking so-called Islamic State.
But quite why Russia apparently chose to bomb this particular US-backed group far to the south in Syria, near to where the Iraqi, Syrian and Jordanian borders meet, still remains something of a mystery.