Officials in Washington are increasingly worried that Russia is preparing to invade Ukraine.
Fears of a cross-border incursion intensified Tuesday after reports that a convoy of 287 Russian trucks was headed to the border.
While the Kremlin insisted the convoy was loaded with humanitarian aid, officials with NATO and the Obama administration expressed fears that the trucks might be something more.
"If this is some sort of Trojan Horse, it would be illegal," said Pentagon spokesman Army Col. Steve Warren on Tuesday.
“We are concerned that Russia could try to use a humanitarian or peacekeeping operation as a pretense for inserting elements of its military force into Ukraine,” another U.S. official told The Hill.
Russian President Vladimir PutinVladimir Vladimirovich PutinOvernight Defense & National Security — Quick vote on defense bill blocked again Kremlin claims Ukraine may try to win back rebel-controlled regions by force Blinken threatens coordinated sanctions on Russia over Ukraine MORE would have a substantial military force to call upon should he choose to order an invasion.
A NATO military officer said there are about 20,000 Russian forces massed along the southeastern border of Ukraine, including tanks, infantry, artillery, air defense systems, logistic troops, special forces and various aircraft.
Ukrainian officials, meanwhile, estimate that that there are a total of 45,000 troops, 160 tanks, 1,360 armored vehicles, 390 artillery systems, 150 truck-mounted ground-to-ground rocket launchers, 192 fighter jets and 137 helicopters. Those numbers include Russian forces in Crimea, the NATO official said.
With the public’s attention in the U.S. focused on Iraq, NATO Secretary-General Anders Fogh Rasmussen warned Monday that there was a "high probability" Russian forces could begin marching across the border.
An invasion would be a major gamble for Putin, who has already been isolated on the world stage by the downing of a Malaysia Airlines jet. The U.S. says separatists in Ukraine, who had received training and arms from Russia, brought down the plane.
With trust in Putin at an all-time low, U.S. and Western officials have said Russia should deliver aid to eastern Ukraine under tight conditions.
The State Department said it would support the aid shipments only if they were received at a border crossing point controlled by Ukraine, passed through appropriate custom clearances and then given to the Red Cross.
"Russia has no right to move into Ukraine unilaterally, whether under the guise of humanitarian convoys or any other pretext, without Kiev's permission," said Marie Harf, deputy spokeswoman at the State Department, during a press briefing Tuesday.
When asked whether the convoy was an invasion, Harf replied, “Well, we don't know.”
“We do have concerns. And that's why, as we've said today, if it goes through all of these steps, then we would support this,” she said.
NATO spokeswoman Oana Lungescu echoed that line.
"Without the formal, express consent and authorization of the Ukraine government any humanitarian intervention would be unacceptable and illegal," she said, adding that officials were "following the situation closely."
Experts said the aid mission could be laying the groundwork for a de facto invasion, as was the case in March when Russia annexed Crimea without firing a shot.
"This humanitarian mission is part of an incremental process that could lead to an eventual invasion," said Luke Coffey, the Margaret Thatcher fellow at the Heritage Foundation.
Putin's move comes as Ukrainian forces are close to defeating pro-Russian separatists in the eastern cities of Donetsk and Luhansk, where both sides are reportedly considering a cease-fire.
"It's part of a plan. Rebels say we want a cease-fire, then Putin brings in 'peacekeepers' to enforce the cease-fire," Coffey said.
The maneuvering comes just weeks after Russian separatists shot down the Malaysian civilian airliner, killing all 298 people on board. An investigation into the incident has not yet been concluded, but U.S. military and intelligence officials say the missile that downed the plane was fired from rebel-controlled territory.
Despite being hit with a new round of sanctions from the U.S. and Europe, Russia is continuing to aid, equip and train the armed separatists, according to a NATO military officer.
“Rockets have been launched at targets in Ukraine from Russian territory. This interference must stop, and Russia must use its leverage to deescalate the situation and end the flow of weapons, fighters and money across its border,” the officer said.
Sen. Kelly Ayotte (R-N.H.), a member of the Senate Armed Service Committee, said Putin's intentions are clear.
“As Russian combat forces amass on the Ukrainian border and reportedly fire artillery on Ukrainian forces from Russian territory, Putin is attempting to use a so-called aid convoy to portray the Kremlin as a responsible provider of humanitarian relief," she said.
Ayotte and other lawmakers have called for the White House to provide military aid to Ukraine, including anti-aircraft weapons. The administration has said it is focusing on providing nonlethal aid, in order to avoid inflaming the situation.
“Putin has instigated, fueled and perpetuated the crisis in eastern Ukraine," Ayotte said. "If Putin is truly concerned about the well-being of Ukrainians in Donetsk, he could end this crisis by stopping the flow of Russian fighters and weapons to Ukraine."
Coffey said the U.S. lacks a strategy to deal with Russian aggression.
"We’ve gone from 'reset' to regret. We need a strategy when what we have is a response. We don’t have a strategy like Putin does," he said.