U.S. Secretary of State, John Kerry has been positively badgering Sergei Lavrov with expressions of concern over reports of a Russian military build-up in Syria, and warnings over the disastrous consequences of Moscow continuing to back its long-time Syrian ally, Bashar al Assad.
The latest warnings from Washington -- so far unheeded by Moscow -- come as satellite images of a Syrian government-held air base at Latakia appear to confirm an expansion of Russia's military footprint.
U.S. officials say they believe Russia may be constructing a forward air operating base there, though no Russian fighter aircraft have, as yet, been deployed, according to the Pentagon.
The Kremlin denies expanding its presence in Syria, but makes no secret of its military assistance to the government of the country, which it sees as a bulwark against extremist groups like ISIS.
Speaking in Dushanbe, Tajikistan, at a regional security conference this week, Russia's President, Vladimir Putin, made an impassioned defense of his support for the Assad regime, encouraging other countries to do the same.
"We support the Syrian government in fighting the terrorist aggression. We have been and will be providing all necessary military-technical assistance and we call on other countries to join us," Putin told the delegates.
"If Russia hadn't been supporting Syria, the situation there would be worse than in Libya and we would see more refugees," he added.
But it would be naïve to see Russia's apparently unflinching support for the Syrian government in terms of anything other than self-interest.
For one, the Kremlin is genuinely concerned about the consequences for its own security if Islamist rebels were to make further gains in Syria, or even take control of the country.
Russia has an Islamist insurgency of its own, in the North Caucasus region, which includes Chechnya, and there are fears an ISIS victory in Syria may have reverberations there. Some of the top military commanders of ISIS are Russian speakers of Chechen origin.
Secondly, Russia has commercial and military interests in Syria it very much wants to keep, but could lose in any Syrian regime change. These include, but are not limited to, a naval facility at Tartus, Russia's only Mediterranean toe-hold. Russia has billions of dollars of commercial investments in Syria too, including oil and gas infrastructure, which it wants to protect.
But practical concern isn't what is really at play. Russia's support for Syria has become the latest front in a wider battle being fought by the Kremlin for influence on the international stage.
Just as the Kremlin believes allowing Eastern Ukraine to be governed by pro-Western authorities in Kiev would spell the end of Russian influence in Ukraine, it also assesses Syria in a similar vein.
If Syria falls, so too will Russian influence in the Middle East, already eroded by the U.S.-led invasion of Iraq and the chaos in Libya.
Its uncompromising support for Assad and apparent willingness to put boots on the ground -- just as it is accused of doing in Eastern Ukraine -- may shift the military balance in favor of Assad.
But it will also send a strong message: that Russia, despite falling oil prices and Western sanctions, is still a force to be reckoned with.