The United States says Russia's latest movements at an airfield in Syria suggest it is planning to establish a forward air-operating base in Syria.
Pentagon spokesman Captain Jeff Davis said on September 14, "We have seen movement of people and things that would indicate that they plan to use that base there, south of Latakia, as a forward air-operating base."
Davis declined to offer specific details from U.S. intelligence reports about the number of Russian troops there or say what kinds of military equipment have arrived.
But Reuters quoted U.S. military officials, who would only speak on condition of anonymity about U.S. military intelligence reports, as saying that seven Russian T-90 tanks had been sighted at the airfield.
The Britain-based Syrian Observatory for Human Rights said on September 13 that its observers in Syria had confirmed that "Russian forces are building a long runway capable of accommodating large aircraft near the Hemeimeem military airport in Syria's coastal province of Latakia."
Russia, a key ally of Syrian President Bashar al-Assad, says its increased military activity -- including the deployment of Russian military specialists to train Syrian forces -- is part of the international effort to defeat terrorism.
Foreign Minister Sergei Lavrov confirmed in an interview on a Russian television news program on September 13 that Russian military weaponry was being sent to Syria.
Lavrov said the shipments "are ongoing and they will continue."
Lavrov also said that those shipments "are inevitably accompanied by Russian specialists, who help to adjust the equipment [and] to train Syrian personnel how to use this weaponry."
Despite Lavrov's public statements, Syria's ambassador to Russia, Riyad Haddad, told Interfax on September 14 that reports of Russian troops in Syria were "a lie spread by Western countries, the United States."
Pentagon spokesman Davis said on September 14 that Washington had concerns about ensuring that any Russian military air operations in Syria not come into conflict with U.S. and coalition air strikes that are being conducted in other parts of Syria against Islamic State (IS) targets.
Nine countries in the U.S.-led coalition have taken part in air strikes against IS militants in Syria since September 2014.
They include the United States, Britain, Canada, Bahrain, Jordan, Qatar, Saudi Arabia, the United Arab Emirates, and Turkey.
Australia, the Netherlands, and France have also carried out air strikes against IS militants in neighboring Iraq.
French President Hollande said on September 14 that air strikes by French warplanes also "will be necessary in Syria."
With reporting by Reuters, AP, AFP, BBC, and Interfax