The U.S. warned Moscow on Wednesday that the Russian military's increasing presence in Syria risked escalating the bitter conflict that has ravaged the Middle Eastern country for the past four years, killing hundreds of thousands of people and displacing millions. The warning came in a phone call from U.S. Secretary of State John Kerry to his Russian counterpart, Sergei Lavrov. It was the latest in a series of diplomatic engagements designed to underscore U.S. concerns about Russia's role in Syria.

During the call, Kerry "reiterated our concern about these reports of Russian military activities -- or buildup, if you will -- in Syria and made very clear our view that, if true and borne out, could lead to greater violence and even more instability in Syria," State Department spokesman John Kirby told reporters.

In recent weeks, a wide variety of Russian military equipment and personnel have reportedly been seen arriving at the normally quiet Tartus naval base in eastern Syria, which Russia has maintained since the 1970s. They include cargo aircraft, tank landing ships and naval infantry, the equivalent of the U.S. Marines. In addition, reports surfaced this week of Russian forces constructing housing units at a Syrian airfield, which already had facilities for 1,000 people, suggesting that more personnel may be expected.

Russia's foreign ministry previously complained of a “strange hysteria” over its activities in Syria. Moscow said its personnel in the country were “military experts ... who are instructing (the Syrians) on the use of the military systems being delivered" to Syrian President Bashar Assad's forces, Russian Foreign Ministry spokeswoman Maria Zakharova said in a statement, cited by CNN.

Russia has a long commercial relationship with the Assad regime. According to some 2012 estimates, 10 percent of Russia's global arms sales go to Syria, with contracts in that year estimated to be worth $1.5 billion.

Unnamed U.S. sources told Reuters on Wednesday that Russian forces had begun participating in military operations in Syria, in support of the Assad regime. Other sources have not confirmed the report which, if true, would represent a significant escalation of Russia's role in the conflict -- and could have an impact both in Syria and beyond. U.S. officials added that the intent of Russia's moves in Syria was still unclear.

In addition to the U.S., NATO chief Jens Stoltenberg expressed concern Wednesday about Russia's role in the Syrian conflict.

The U.S. has, in the past, called for Assad to step down and has provided limited support to moderate Syrian rebels fighting the Islamic State group, which is also fighting against Assad's forces.