"We have seen indication in recent days that Russia has moved people and things into the area around Latakia, and the airbase there," Pentagon spokesman Capt. Jeff Davis said Monday.
Latakia, a port city on Syria's Mediterranean coast, is in an area controlled by the forces of Syrian President Bashar al-Assad, a close Russian ally.
The United States has been monitoring Russia's recent movements of military personnel in Syria with unease.
Last week, U.S. Secretary of State John Kerry called Russian Foreign Minister Sergei Lavrov
to warn him that Moscow's reported troop buildup in Syria could intensify the vicious civil war there.
The Pentagon on Monday declined to disclose specific intelligence on what it had observed around Latakia, but Davis said the U.S. military hasn't so far seen Russian fighter jets, helicopters, or gunships arrive in the area.
He said the movements are "progressing on a daily basis."
The news agency Reuters cited two unidentified U.S. officials as saying that Russia had positioned about a half dozen tanks at an airfield near Latakia.
Russian intentions murky
U.S. officials have been trying to determine the true intent behind the Russian moves in Syria.
They say the possibilities range from preparations to attack positions of the militant group ISIS to the more likely scenario of attacking moderate rebel forces who are fighting al-Assad's military.
Another possibility is that the movements are advance efforts to help Russia influence events inside Syria if al-Assad's regime falls.
Russia's Foreign Ministry said last week that there are "Russian military experts in Syria who are instructing (the Syrians) on the use of the military systems being delivered" to al-Assad.
Russia "has long been supplying arms and military equipment to Syria in accordance with bilateral contracts," the ministry said.
Russian President Vladimir Putin said Tuesday that Russia is supporting the Syrian government in its fight against ISIS' "terrorist aggression."
"We've been providing and will be providing all necessary military-technical assistance and we call on other countries to join us," he said in comments aired on Russian state TV from an event in Dushanbe, Tajikistan.
"If Russia hadn't been supporting Syria, the situation there would be worse than in Libya and we would see more refugees," Putin said.
Differing goals in tangled conflict
The complex Syrian conflict has raged for more than four years, taking the lives of more than 200,000 people.
Rebel groups have been trying to defeat al-Assad's forces, but he has been able to retain his grip on parts of the country with help from Iran and Russia.
The Sunni Muslim extremists of ISIS have capitalized on the chaos, imposing their brutal rule on large areas of northern and eastern Syria.
Both Moscow and Washington are opposed to ISIS, but their broader aims differ.
A U.S.-led coalition is conducting a bombing campaign against ISIS positions in Syria and Iraq.
The United States has also been offering limited support to moderate Syrian rebels in the fight against ISIS. Those rebels, though, are also trying to drive out al-Assad, whom U.S. President Barack Obama has called on to step down.
Backing al-Assad 'not a winning strategy'
On Monday, the U.S. ambassador to the United Nations said Russia's support for al-Assad "is not a winning strategy."
"Doubling down on a regime that gases its people, that barrel bombs its people, that tortures people who it arrests simply for protesting and for claiming their rights -- that's just not going to work," Samantha Power told CNN's Christiane Amanpour
Davis, the Pentagon spokesman, said Monday that U.S. officials are concerned about increased air traffic in the skies above Syria.
He noted, though, that incoming Russian flights have so far followed civilian routes, which are already clear of coalition operations in Iraq and Syria.