DONETSK, Ukraine — The steadily advancing Ukrainian army is setting its sights on the largest rebel-held city in eastern Ukraine, while Western officials warned Wednesday 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 back into 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 Putin’s power.
Russia already is showing signs of economic dismay from sanctions imposed this year, but Putin on Wednesday showed Moscow aims to fight back, calling on government agencies to develop a list of agricultural imports from sanctions-imposing countries that could be banned for up to a year.
‘‘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,’’ US Defense Secretary Chuck Hagel said Wednesday. US 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, an assertion the Russian government has repeatedly denied.
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 over rebel-held territory on July 17, 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.
Prime Minister Donald Tusk of Poland said he believed ‘‘the threat of a direct intervention [by Russia] is definitely greater than it was a few days ago, or two weeks ago.’’
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 on Thursday 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 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, Putin also faces agitation from nationalists who want Russia to take more assertive action.
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 airstrikes. 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 during the past few days. The claim could not be independently verified.