The Obama administration's hopes for Syrian President Bashar Assad to step aside and make way for a democratic government in Damascus are being dashed once again -- this time, by evidence that the dictator's strongest ally is joining the fight to keep him in power. 

According to multiple reports, Russian troops are on the ground to aid Assad forces battling Islamic State militants. 

To make matters worse, the Obama administration's push to train and equip a moderate rebel force has failed to get off the ground. The developments are, in turn, fueling criticism of the U.S. effort to bring peace to the long-running civil war. 

"The whole region is in chaos," Sen. John McCain, R-Ariz., told Fox News on Thursday. 

The State Department has warned that Russia's latest involvement in Syria could "further escalate the conflict" and "risk confrontation with the anti-ISIL Coalition operating in Syria." 

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But Moscow appears to have ignored that warning, despite a second call in five days from Secretary of State John Kerry to his counterpart in Russia expressing "concerns" about the "buildup." 

Reuters now reports that the Russian forces, according to Lebanese sources, are participating in military operations to boost Assad troops. Israel's defense minister also said Thursday that Russian troops are in Syria to help Assad fight ISIS, and could be looking to launch jets and combat helicopters, the Associated Press reported. 

Russian Foreign Minister Sergey Lavrov acknowledged at a news conference Thursday that Moscow indeed is sending military equipment, which he said falls under "existing contracts." He said Russia "will continue aiding the Syrian government in equipping the Syrian army with all that is necessary for it to prevent a repetition in Syria of the Libyan scenario and other sad events that have occurred in this region, because of an obsession by some of our western partners with ideas of changing unwanted regimes.'' 

Russia earlier acknowledged it has military experts on the ground, but had not addressed the scope of that effort. 

U.S. officials also tell Fox News the Pentagon has observed several Russian military cargo flights into Syria -- to date, the Pentagon has seen seven An-124 "Condor" flights to an airbase outside of Latakia, a port city on the Mediterranean, Fox News is told. 

A separate official confirmed that two Russian Navy tank landing ships moored in Tartus -- where the Russians have a naval base -- in the past few days, offloading armored vehicles as well as nearly 50 Russian marines. 

Asked what the U.S. military planned to do about the Russian buildup, the official said, "nothing." 

It's unclear how far Russia is preparing to go to boost Assad, but one U.S. official told Fox News the presence there is "unprecedented." 

"This is definitely a build-up straight out of Russia's military doctrine," one official said. 

The development comes as the Obama administration's effort to train and equip a moderate rebel fighting force in Syria falters. 

The Pentagon says it has spent nearly $42 million -- out of $500 million allocated by Congress -- to train a total of 54 fighters so far. 

But Politico reported Thursday that all those fighters are now either dead, captured or missing. Asked by Fox News how many of the 54 the Pentagon could account for, a Pentagon spokesman did not answer directly, saying only that they've "been candid that the initial phase of this program did not lift off with the kind of efficiency that we'd hoped." 

The spokesman added that "supporting these moderate Syrian forces is a critical component in the fight going forward." 

Without a credible moderate rebel force operating inside Syria, though, Assad is potentially poised to strengthen his position, now that the Russians are involved. Despite the Obama administration's fizzled "red line" threat over Assad's chemical weapons use and declarations that he needs to relinquish power, Assad's most powerful opponent remains the Islamic State and other terror groups -- which the United States also is fighting. 

McCain said the Russians are moving to "prop up" Assad and argued the Syrian strongman is surviving with the help of allies like Moscow and Tehran. 

"That's the only thing that's keeping Bashar Assad in power," he told Fox News on Thursday. 

He faulted the Obama administration for both withdrawing forces from Iraq and not doing more from the start to get rid of Assad, saying this fueled the rise of ISIS. 

"When we create that vacuum, then it is filled by evil," McCain said. 

Sen. Lindsey Graham, R-S.C., a Republican presidential candidate, also told Fox News that the administration should have helped the Free Syrian Army three years ago by establishing a no-fly zone -- "when it would have mattered." 

Even Democratic presidential candidate Hillary Clinton critiqued the Syria policy. "We have to bring sufficient pressure on Assad to force a political solution in Syria, including a meaningful increase in our efforts to train and equip the moderate Syrian opposition, something I called for early in the conflict," Clinton, the former secretary of state, said Wednesday in Washington. 

State Department spokesman John Kirby told reporters Wednesday that Kerry is warning his Russian counterpart, Lavrov, that Russia is fueling instability in the country where a four-year civil war has claimed 250,000 lives. 

"[Kerry] reiterated our concern about these reports of Russian military activities -- or buildup, if you will -- in Syria and made very clear our view that [it] ... could lead to greater violence and more -- even more instability in Syria," Kirby said. 

Fox News' Lucas Tomlinson and Jennifer Griffin contributed to this report.