Mr Obama on Friday met King Salman of Saudi Arabia to repeat their demand that any lasting settlement in Syria would require an end to the Assad regime.
It leaves the US and Russia implacably opposed in their visions for Syria.
John Kerry, Secretary of State, telephoned his Russian counterpart to express US concerns on Saturday.
"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-Isil coalition operating in Syria," the department said.
The new US details came in the week that Vladimir Putin gave his strongest admission yet that Russia was already providing some military and logistical support to Syria.
"We are already giving Syria quite serious help with equipment and training soldiers, with our weapons," he said during an economic forum in Vladivostok on Friday, according to the state-owned RIA Novosti news agency.
Until now, Russia's backing has included financial support, intelligence, advisers, weapons and spare parts. Mr Putin insisted it was "premature" to talk of a direct intervention.
However, images emerged last week that appeared to show a Russian fighter jet operating over Syrian soil and videos of combat troops speaking the Russian language.
Syrian state television showed images of an advanced Russian-built armoured personnel carrier, the BTR-82a, in combat. Videos also began circulating in which troops shouted orders to one another in Russian.
Last week the Israeli daily Yedioth Ahronoth cited Western diplomatic sources saying that Russia was on the verge of deploying "thousands" of troops to Syria to establish an airbase from which the Russian air force would fly combat sorties against Isil.
Those details appear to be backed by satellite images of a Russian base under construction near Latakia, according to anonymous intelligence officials quoted by several American newspapers.
"If they're moving people in to help the Syrian government fight their own fight, that's one thing," one told the Los Angeles Times. "But if they're moving in ground forces and dropping bombs on populated areas, that's an entirely different matter."
Moscow increasingly justifies its support for the Assad regime by pointing to the rise of violent jihadists in Syria.
The Islamic State of Iraq and the Levant (Isil) has captured a swath of territory since Arab Spring protests in 2011 provoked a heavy-handed regime crackdown.
The conflict is one of the key drivers for the wave of refugees arriving in Europe. It was from Kobane that Aylan Kurdi and his family set out for Europe. The discovery of three-year-old's body on a Turkish beach this week has provoked a change of attitudes towards migrants.
This week, Isil stepped up its programme of cultural cleansing, blowing up temples in the historic city of Palmyra.
And fresh clashes along the border with Turkey claimed the lives of 47 fighters at the weekend, according to the Syrian Observatory for Human Rights.
Syria is already home to Russia's only base outside the former Soviet Union - a naval station in Tartus.
The reported build-up of military activity, centred on Latakia and Idlib province, is in areas dominated by the Alawite sect, which counts President Assad among its number.
