DONETSK, Ukraine -- The steadily advancing Ukrainian army is setting its sights on the largest rebel-held city in eastern Ukraine, while Western officials on Wednesday warned that a Russian military buildup on Ukraine's border could herald a major incursion to protect the separatists.
President Vladimir Putin has resisted mounting pressure from Russian nationalists to send the army in to back the mutiny in eastern Ukraine. Even though the United States and NATO would be unlikely to respond militarily, the West would be certain to impose major sanctions that would put the shaky Russian economy on its knees -- and could quickly erode Mr. Putin's power. Russia already is showing signs of economic dismay from sanctions imposed earlier this year, and President Barack Obama on Wednesday said U.S. sanctions against Russia are straining the country's economy.
"When you see the buildup of Russian troops and the sophistication of those troops, the training of those troops, the heavy military equipment that's being put along that border -- of course, it's a reality; it's a threat; it's a possibility -- absolutely," U.S. Defense Secretary Chuck Hagel said Wednesday. U.S. and NATO officials say there are now about 20,000 Russian troops massed just east of Ukraine.
Pro-Russian separatists in eastern Ukraine have been fighting the Kiev government since April. Ukraine and Western countries have accused Moscow of backing the mutiny with weapons and soldiers, a claim the Russian government has repeatedly denied. Moscow also has denied any buildup on the border.
The West has also accused Russia of most likely providing the insurgents with surface-to-air missiles that may have been used to shoot down a Malaysia Airlines passenger jet July 17 over rebel-held territory, killing all 298 people on board. The prime minister of The Netherlands, whose nationals made up more than half of the victims, said Wednesday that the search for victims' remains is being halted because fighting in the area of the crash site makes it too dangerous to continue.
Polish Prime Minister Donald Tusk said he believed that "the threat of a direct intervention [by Russia] is definitely greater than it was a few days ago, or two weeks ago."
A U.S. official said in an interview that U.S. intelligence shows Russian forces continue to shell Ukrainian positions from inside Russian territory and send heavy weaponry -- including artillery, armored vehicles and air defense equipment -- from a separatist training facility in southwest Russia. The official was speaking on condition of anonymity because he wasn't authorized to discuss intelligence matters publicly.
Adding to the concern is Russia's proposal in recent days for a humanitarian mission to eastern Ukraine. "We share the concern that Russia could use the pretext of a humanitarian or peacekeeping mission to send troops into eastern Ukraine," NATO spokeswoman Oana Lungescu said in an e-mailed statement.
The Ukrainian Foreign Ministry said NATO Secretary-General Anders Fogh Rasmussen will visit Kiev today to meet President Petro Poroshenko and other officials.
Humanitarian concerns are rising as Ukrainian forces come closer to encircling the city of Donetsk and continue their fight against the pro-Russia rebels in the large city of Luhansk. Moscow has pushed for a cease-fire in the east, but the Ukrainian government has appeared bent on riding the momentum of a series of recent military advances to crush the rebels.
While an overt military move into Ukraine would be deeply risky for Russia, Mr. Putin also faces agitation from nationalists who want Russia to take more assertive action.
Aleksandr Dugin, a prominent nationalist ideologue, wrote on his Facebook page this week that Luhansk faces a siege like that of Leningrad in WWII -- an analogy resonating in the heart of his country's patriotic fervor. The nearly 900-day siege by the Nazis is one of Russia's major touchstones of suffering and valor. "Luhansk has to be saved urgently; otherwise, it will be the same baseness as the military aid that wasn't shown" earlier, when nationalists were calling for Russia to intervene, he wrote.
In the Kalininsky neighborhood only 3 miles east of Donetsk's central square, rebels and civilians were milling around outside after a night of what many said they believed were Ukrainian air strikes. There were eight craters at the scene that appeared to be the result of aerial bombing.
In another rebel stronghold, the city of Horlivka about 22 miles north of Donetsk, the city council said in Wednesday's statement that 33 civilians have been killed and 129 wounded by shelling over the past few days. The claim couldn't be independently verified.
As the Ukrainian military intensified its campaign against the rebels, heavily populated areas have increasingly come under attack. Kiev adamantly denies launching artillery barrage and air raids against residential neighborhoods and accuses the rebels of firing at civilian areas. The government has offered little evidence to prove its claims, which Human Rights Watch and others have questioned.