The Ukrainian military is increasing pressure on the separatist stronghold of Donetsk, officials said Saturday, even as concern grows about deeper Russian involvement in the conflict.
The army had advanced to the edge of Horlivka, they said, which is a key shipping point for separatists about 25 miles to the northeast.
"We have approached Horlivka," said Andriy Lysenko, spokesman for the National Security and Defense Council. "Donetsk will be next."
The news comes as the U.S. accuses Russia of moving significant amounts of weaponry across the border, and building up forces on the Russian side of the frontier.
A fierce battle has been waged for weeks in Donetsk, particularly near the airport, which is a supply lifeline for much of the region, known as the Donbass. Those in the area report sounds of gunfire and shellings regularly. Lysenko said Saturday that the country aims to wage an urban war in Donetsk with few if any missile or air attacks.
The conflict there has already exacted a price, however, particularly in civilian casualties. On Thursday, Human Rights Watch released a report that said Grad rockets used by the Ukrainian army killed at least 16 civilians in Donetsk over the last two weeks. Fighting around the city has also thwarted Dutch and other investigators hoping to reach the crash site of Malaysia Airlines Flight 17 several miles to the east.
The conflict also could be leading to tension far from the battle zone. On Saturday, an unknown assailant lobbed a grenade at the home of the mayor of Lviv, the strongly European-leaning city in western Ukraine. The mayor was not home.
The Ukrainian army retook the cities of Lysychansk and Slovyansk to the north of Donetsk this month. The success in Slovyansk prompted President Petro Poroshenko to proclaim a turning point in the campaign.
Donetsk would be an even bigger victory. The city of 1 million is at the epicenter of the pro-Russia insurgency. Though separatists would probably retreat to a few urban centers and continue to fight, most experts say that Ukrainian forces could triumph relatively quickly if Donetsk fell.
But Russia remains a wild card. The Pentagon says Moscow has been transferring more heavy weaponry to separatists. A spokesman said Friday that the U.S. believed Russia was set to move up to a dozen 200-millimeter rocket launchers to separatists, some of the strongest weaponry it has supplied to date. Russia has also increased its troop count along the border to 12,000, the Pentagon said.
Deeper Russian involvement could make the conflict drag on, experts say.
U.S. officials also say Russia has launched artillery attacks across the border at Ukrainian forces. The Ukrainian government, meanwhile, has suggested that two fighter jets lost over eastern Ukraine last week may have been downed by missiles fired from Russian territory. Lysenko said Saturday that four Ukrainian soldiers were killed in skirmishes along the border.
Critics also point to what they say are deep ties between insurgency leaders Oleksandr Boroday and Igor Girkin, known as Strelkov, and the Russian security establishment.
Russian President Vladimir Putin has denied supplying weapons to separatists or engaging in military activity along the Ukrainian border. The Russian Foreign Ministry on Friday called the recent U.S. intelligence reports of Russian military activity "an unrelenting campaign of slander."
Sen. John McCain (R-Ariz.) and other Republican congressional leaders have called for the U.S. to furnish Ukraine with weapons, and some hard-line voices have advocated a greater sharing of intelligence with Kiev. The U.S. has led a push for harsher sanctions against Russia and shipped food and other supplies to the Ukrainian army, but has otherwise stayed out of the conflict.