Russian, Syrian and Iranian military commanders have set up a coordination cell in Baghdad in recent days to try to begin working with Iranian-backed Shia militias fighting the Islamic State, Fox News has learned.
Western intelligence sources say the coordination cell includes low-level Russian generals. U.S. officials say it is not clear whether the Iraqi government is involved at the moment.
Describing the arrival of Russian military personnel in Baghdad, one senior U.S. official said, "They are popping up everywhere."
The Russians already have been building up their military presence in Syria, a subject expected to factor prominently in a planned meeting between President Obama and Russian President Vladimir Putin in New York Monday on the sidelines of the U.N. General Assembly.
While the U.S. also is fighting the Islamic State, the Obama administration has voiced concern that Russia's involvement, at least in Syria, could have a destabilizing effect.
Moscow, though, has fostered ties with the governments in both Syria and Iraq. In May, Iraqi Prime Minister Haider al-Abadi flew to Moscow for an official visit to discuss potential Russian arms transfers and shared intelligence capability, as well as the enhancement of security and military capabilities, according to a statement by the Iraqi prime minister's office at the time.
Meanwhile, a U.S. official described to Fox News how, over the weekend, the Russians were able to move 24 attack jets into Syria undetected.
The Russian military flew 12 Sukhoi Su-25 "Frogfoot" and a dozen Sukhoi Su-24 "Fencer" attack aircraft in "tight formations" under the "steady stream" of the large Russian An-124 cargo planes that have been ferrying supplies from bases in Russia through Iran before traveling on to Syria, the official said.
The large cargo planes appeared as "a big blip" on radar, but flying beneath them were "tight formations" of the smaller Russian fighter jets that used jamming pods and switched off their IFF, which would identify the aircraft to radar.
The large Russian cargo planes have the capability to fly directly from Russia to Syria, but the smaller attack aircraft do not.
"The Russian jets did not have the legs to make it directly from Russia to Syria, and needed a base to refuel," said the official, who spoke to Fox News under the condition of anonymity because he was not authorized to disclose sensitive information.
According to the Aviationist, the Russian cargo planes and fighter jets landed at an airbase in Hamadan, Iran, roughly halfway between Baghdad and Tehran on Sept 18-19.
Fox News also has learned from U.S. military sources that the Russians have begun flying some of the Sukhoi fighter and attack jets from Bassel al-Assad airport, in Latakia, now a Russian forward operating base along the Mediterranean.
The planes are not dropping bombs or conducting attack missions, but just flying around near the base, according to one official. The official also confirmed that Russian destroyers are in position off the Mediterranean coast.
On Thursday, State Department spokesman John Kirby denied a U.S. intelligence failure led to U.S. officials being caught unaware of the two dozen Russian warplanes arriving in Syria.
"I can tell you that we've been watching this very, very closely ... and we have not been ignorant of what the Russians have been doing," said Kirby.
Asked Thursday about Russia's military involvement in Syria, Defense Secretary Ash Carter cautioned that without Russian support for a "political transition" in Damascus, it could "pour gasoline on the ISIL phenomenon rather than to lead to the defeat of ISIL."
But just two days ago, Secretary of State John Kerry said the Russian build-up was consistent with defensive measures.
"For the moment, it is the judgment of our military and most experts that the level and type represents basically force protection, a level of protection for their deployment to an airbase given the fact that it is in an area of conflict,'' Kerry said at the State Department Tuesday.
This week, former CIA director Gen. David Petraeus testified on Capitol Hill, warning that inaction in Syria carries risks for the United States.
"Russia's recent military escalation in Syria is a further reminder that when the U.S. does not take the initiative, others will fill the vacuum -- often in ways that are harmful to our interests," Petraeus said.
-- Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel.
-- Jennifer Griffin currently serves as a national security correspondent for FOX News Channel . She joined FNC in October 1999 as a Jerusalem-based correspondent.