"They could start at any moment," the official said. "They are ready."
After several days of Russian familiarization flights, there is no reason they could not begin, the official added. And Russian drones have been collecting potential targeting information in their flights. But the U.S. doesn't know what the Russians have in mind and when they will make a decision on airstrikes.
Four Russian SU-34 Fullback fighters are now at the Latakia air base, and more than 600 Russian troops are in place.
Pentagon spokesman Peter Cook told reporters Tuesday that Secretary of Defense Ashton Carter directed his staff to "open lines of communication with Russia on de-confliction."
The timing of these discussions is to be worked out in the coming days. The purpose of the discussions is "to ensure the safety of coalition air crews," he said.
Cook added that the two nations have common ground when it comes to fighting ISIS, also known as ISIL, with Carter making clear that "the goal should be to take the fight to ISIL and not to defend the Assad regime."
Russia continues to position itself to potentially launch airstrikes in Syria -- but their movements suggest that their targets are something other than ISIS, according to U.S. officials.
"We see some very sophisticated air defenses going into those airfields. We see some very sophisticated air-to-air aircraft going into these airfields. I have not seen ISIL flying any airplanes that require SA15s or SA22s. I have not seen ISIL flying any airplanes that require sophisticated air-to-air capabilities," Gen. Phillip Breedlove, NATO's Supreme Allied Commander, said on Monday.
"I'm looking at the capabilities and the capacities that are being created and I determine from that what might be their intent. These very sophisticated air defense capabilities are not about ISIL. They're about something else," he concluded.
Separately, Deputy Defense Secretary Robert Work confirmed to the Senate Tuesday that Russia has violated a missile treaty with the United States but indicated that the administration didn't plan to take any action at present.
New Hampshire Republican Sen. Kelly Ayotte asked Work at a hearing whether he thought the Russians had breached the Intermediate-Range Nuclear Forces Treaty governing the elimination of medium-range missiles.
"We believe very strongly that they did," he responded. But he also said, "This is still in discussions and we have not decided on any particular action at this point," noting that the U.S. has been in contact with the Russians over the issue.
Meanwhile, America's own efforts to turn the tide in Syria have faltered once again.
The Pentagon has stopped moving new recruits from Syria to its training centers in Jordan and Turkey to train and equip moderate rebels to fight ISIS, a U.S. defense official told CNN Tuesday. The deeply troubled program has several dozen fighters in training right now, but new fighters will not be brought in until the White House makes a decision on how it wants to change the effort.
Several options are being considered, including having the rebels train to provide assistance in calling in airstrikes and conducting communications.
The official described the action as a "pause" in the effort until a way ahead is decided. Also paused for now is vetting of Syrians who say they want to join the effort.
Russian President Vladimir Putin mocked the stumbling train-and-equip program in his address to the United Nations Monday.
Putin and Obama, who were each confrontational toward the other in their morning speeches, then met to discuss Syria and Ukraine later in the day.
"We have clarity on their objectives," one senior administration official said after the meeting. "Their objectives are to go after ISIL and to support the government."
Defense officials have previously told CNN that the U.S. believes Moscow may fear that Syrian President Bashar al-Assad may not be able to retain power in the war-torn country and wants to be in position to be able to support a proxy should the situation collapse.
Russia is also a close ally of Assad and may want to bolster him, while the U.S. has repeatedly called for him to go in order to resolve the five-year civil war.
U.S. intelligence has observed the possible staging of multiple rocket launch systems at Russian ports, which could be loaded onto ships bound for Syria. It has also observed a small number of Russian long-range bombers being moved into airfields in the southern part of Russia, which could give them a long-range heavy bomber capability in Syria, according to the officials.
Syria and Russia have also established a coordination center in the Hamah province, though its purpose remains unclear, officials said.
Moscow is also sending drones in the southwest Idlib, Hamah and Latakia areas to gather reconnaissance and intelligence -- though none of those regions are near ISIS positions.
The new developments continue a pattern of strategic buildup for Russia in and near Syria. In recent weeks, Russia has moved aircraft, tanks, artillery and armored vehicles into Syria, though its objective in the region remains murky.
The intelligence on Russia's actions make clear that the U.S. is dedicating a substantial amount of satellite and eavesdropping capabilities to monitoring the developments around the clock.