As Russia continues to defend its airstrikes in Syria, denying that civilians have been killed, sources have said that hundreds of troops, allies of President Bashar al-Assad, have arrived in the country within the last week and a half.
Reuters reported a Lebanese source said that Russian "airstrikes will in the near future be accompanied by ground advances by the Syrian army and its allies," with the goal being to recapture territory that had been taken by rebels.
"It is possible that the coming land operations will be focused in the Idlib and Hama countryside," this source told Reuters.
This image taken in Wednesday, Sept. 30, 2015 posted on the Twitter account of Syria Civil Defence, also known as the White Helmets, a volunteer search and rescue group, shows the aftermath of an airstrike in Talbiseh, Syria. Russia on Wednesday carried out its first airstrikes in Syria in what President Vladimir Putin called a pre-emptive strike against the militants. Khaled Khoja, head of the Syrian National Council opposition group, said at the U.N. that Russian airstrikes in four areas, including Talbiseh, killed dozens of civilians, with children among the dead. (Syria Civil Defence via AP)
One of the two sources speaking to Reuters also said that a "vanguard of Iranian ground forces began arriving in Syria: soldiers and officers specifically to participate in this battle. They are not advisors ... we mean hundreds with equipment and weapons." This source, also Lebanese, added that they believe Iraqis would join as well.
U.S. defense officials also confirmed to Fox News that it "has always been understood in this building that the Russians would provide the air force, and the Iranians would provide the ground force in Syria." Another official told Fox Iranians had already been on the ground in Syria. Neither of these sources discussed the size of the force.
Russian jets carried out a second day of strikes in Syria Thursday, and some activists claimed that the targets included rebels backed by the United States as concerns grew about a conflict that has now drawn in warplanes from the world's two most powerful militaries.
Russian President Vladimir Putin denied reports that civilians were killed in any Russian airstrikes.
"We are ready for such information attacks," he said in a live broadcast from the Kremlin. "The first reports of civilian casualties came even before our jets took off."
Sen. John McCain (R-Ariz.) told CNN he could "absolutely confirm" Russian airstrikes were "against our Free Syrian Army or groups that have been armed and trained by the CIA, because we have communications with people there."
In this photo made from the footage taken from Russian Defense Ministry official web site on Thursday, Oct. 1, 2015 a bomb explosion is seen in Syria. Reacting to criticism that it is targeting opponents of the Syrian government, a spokesman for Russian President Vladimir Putin admitted on Thursday that Russia's airstrikes in Syria are targeting not only Islamic State militants but also other groups. (AP/ Russian Defense Ministry Press Service)
McCain went on to say the strikes were "an incredible flouting of any kind of cooperation or effort to conceal what their first — Putin's priority — is. And that is of course to prop up Bashar al-Assad," CNN reported.
Russian Defense Minister Igor Konashenkov said Russian aircraft damaged or destroyed 12 targets in Syria belonging to the Islamic State group including a command center and two ammunition depots. Officials acknowledged, however, that other unidentified groups were being targeted as well.
Konashenkov said Russian Su-25M and Su-25 jets flew 20 sorties between Wednesday and Thursday morning, and he insisted that civilian areas were not targeted.
In this image made from video provided by Homs Media Centre, which has been verified and is consistent with other AP reporting, smoke rises after airstrikes by military jets in Talbiseh of the Homs province, western Syria, Wednesday, Sept. 30, 2015. Russian military jets carried out airstrikes in Syria for the first time on Wednesday, targeting what Moscow said were Islamic State positions. U.S. officials and others cast doubt on that claim, saying the Russians appeared to be attacking opposition groups fighting Syrian government forces. (Homs Media Centre via AP)
The Britain-based Syrian Observatory for Human Rights said airstrikes in the central province of Hama on Thursday hit locations of the U.S.-backed rebel group, Tajamu Alezzah, as well as the province of Idlib, which is controlled by a coalition of rebel groups that include al-Qaida's affiliate in Syria, Jabhat al-Nusra.
The British group said Tajamu Alezzah was also targeted on Wednesday.
Russia's air campaign in support of Syrian government forces began Wednesday in what Putin called a pre-emptive strike against militants. Putin's spokesman, Dmitry Peskov, said Russia was going after IS militants as well as a "list" of other groups.
"These organizations are well known and the targets are chosen in coordination with the armed forces of Syria," he said Thursday, without specifying.
Russian President Vladimir Putin holds a meeting with senior government officials at the Novo-Ogaryovo residence outside Moscow, Russia, on Wednesday, Sept. 30, 2015. Russian military jets carried out airstrikes against the Islamic State group in Syria on Wednesday for the first time, after President Vladimir Putin received parliamentary approval to send Russian troops to Syria. (Alexei Nikolsky/RIA Novosti, Kremlin Pool via AP)
On Wednesday, however, Sergei Ivanov, Putin's chief of staff, said "the operation's target is solely air support for the Syrian government forces in their fight against the ISIS."
Putin has said Russia would be fighting "gangs of international terrorists." The Syrian government considers all rebel groups terrorists.
The Pentagon said it would begin talks Thursday with Russian military officials on ways to avoid U.S. and Russian forces firing on each other in Syria as tensions escalate over the airstrikes. The talks will be held by video teleconference.
On Wednesday, Defense Secretary Ash Carter said the Russians appeared to have targeted areas that did not include IS militants. He complained that Moscow did not use formal channels to give advance notice of its airstrikes to Washington, which is has been conducting its own airstrikes in Syria against IS for more than a year.
Carter said the Russians should not be supporting the Assad government and their military moves are "doomed to fail."
The U.S. defense chief also said the Obama administration was open to holding direct talks with the Russians on "deconflicting" their military operations in Syria, or arranging ways to avoid firing upon each other or creating unintended incidents in the air.
"Our goals for this meeting are the following: to facilitate the flow of information between coalition forces and Russian elements that will help us maintain the safety of our personnel in the region, which is critical; to ensure that any additional Russian actions do not interfere with our coalition's efforts to degrade and defeat ISIL, and to clarify that broader U.S. security commitments in the region remain unchanged," Carter said.
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The Associated Press contributed this report. This story has been updated to include more information.