U.S. officials are expressing growing concern about Russia's military build-up in Assad-controlled Syria, calling it "unprecedented" -- with one telling Fox News it compares in scope to Vladimir Putin's incursion into Crimea.

"It's beginning to look like Crimyria," the official told Fox News.

Two U.S. officials who have reviewed the latest intelligence told Fox News that satellite imagery reveals more flights of massive Russian An-124 "condor" military cargo planes landing in Syria. They are offloading troops, including just under 50 Russian marines, and armored vehicles.

U.S. officials said the Russian activity in Syria is unlike any they've seen since the start of the Syrian civil war four years ago.

"This is definitely a build-up straight out of Russia's military doctrine," said one official.

The Russian government is not denying the build-up.

Russia's Foreign Ministry disclosed early Wednesday that Russian military experts indeed are assisting forces loyal to Syrian President Bashar al-Assad in that country's long-running civil war.

Foreign Ministry spokeswoman Maria Zakharova told Reuters that advisers were assisting with Russian arms deliveries to Syrian government forces, which Moscow says are aimed at fighting Islamist militants. The spokeswoman did not give a precise number of Russian military personnel in Syria.

The exact extent of the build-up is unclear, but a U.S. official told Fox News that the Russians have constructed 100 additional housing units at a base near Latakia, a coastal city along the Mediterranean.

The Pentagon has not seen evidence that the newly arrived Russian troops are engaged in combat.

But the build-up is escalating despite Secretary of State John Kerry calling his Russian counterpart Sergey Lavrov over the weekend to voice "concerns" about Russian military presence.

In a statement released Saturday after Kerry's call with Lavrov, the State Department warned that Russia's continued actions could "further escalate the conflict" and "risk confrontation with the anti-ISIL Coalition operating in Syria."

The statement appeared to mark a change in policy from last week when the Pentagon and State Department initially welcomed Russia's increased involvement in Syria.

Vladimir Putin's government is aligned with Assad, whom the Obama administration wants out of power. But the administration also is boosting local forces fighting the Islamic State in Iraq and Syria (ISIS), and initially cast the stepped-up Russian involvement as contributing toward that anti-ISIS effort.

White House spokesman Eric Schultz said Wednesday that the administration, though, does have deep concerns about reports Russia may have deployed military personnel and aircraft in Syria.

He said the administration is monitoring the situation closely.

Fox News' Lucas Tomlinson and Jennifer Griffin contributed to this report.