The fighter, who asked to be identified only by his first name, Ahmed, to discuss operational matters and negotiations, said he had been at a recent meeting with Russians, Kurdish militias and Arab rebels in Tal Abyad, a small city near the Turkish border.

Ahmed said that his group had previously been supported by Ahmed al-Jarba — a Syrian opposition figure closely tied to Saudi Arabia, which is staunchly opposed to Mr. Assad and Russia’s role in Syria — and that the Russians’ outreach had made the fighters wonder if Saudi Arabia and Russia had a secret understanding.

“We were really surprised to meet a Russian delegation in our headquarters,” he said.

Ahmed said the Russians had offered his tribe, the Shweytat, weapons and assistance to take back their home area in Deir al-Zour from the Islamic State. The Shweytat are bitter enemies of the militant group, which slaughtered hundreds of their people after they refused to submit to its rule. They have contributed to the ranks of Arab forces fighting alongside the Kurdish People’s Protection Units, or Y.P.G., Ahmed said.

But the aid plan was suspended, he said, after the Russians asked him and his comrades to gather 300 fighters and they could not immediately muster more than 200. In any case, he remained skeptical that what the Russians were proposing would be enough to have an impact on the effort to take back Deir al-Zour.

“What can I say?” he said. “It’s very hard to liberate Deir al-Zour with this amount of weapons.”

In the northeast, in Qamishli, a Syrian activist network called the Local Coordination Committees has reported in recent days that 100 Russian military personnel have been deployed at the city’s airport, and that Russian officials met with both government officials and Kurdish militia leaders to discuss deploying forces in the city, the provincial capital. These reports added to accounts that the Kurds, who have what amounts to a nonaggression pact with government forces, coordinate with both sides: American and Arab insurgents on one hand, and the government and Russia on the other. But the activists said Russians were being deployed in areas controlled by government forces, not the autonomous zones carved out by Kurds, where Americans are aiding the Kurdish and Arab militias.

Two Pentagon officials confirmed the details of the Russian deployment, including the critical point that Moscow does not seem to be focused on directly supporting fighters in the same places as the Americans.

“I’m not sure I’d characterize it as providing support to the same people as we are,” Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, said in an interview in Brussels.