The head of the NATO alliance on Thursday urged Russia to “step back from the brink” by withdrawing its troops massed on the Ukrainian border or face greater isolation and debilitating sanctions.

“Do not use peacekeeping as an excuse for war making,” said NATO Secretary General Anders Fogh Rasmussen, pointedly speaking from the Ukrainian capital, Kiev, where he visited to show support.

NATO has warned that Russia could invade Ukraine under the pretext of sending in a peacekeeping mission to stop the war between government troops and pro-Russian separatists. Russia has as many as 20,000 troops at the border. Rasmussen met with Ukrainian officials, offering intensified NATO assistance to the country’s military in the form of advisers, training and logistical support.

“I have no doubt the international community would react decisively, with broader, deeper, tougher economic sanctions if Russia intervenes further,” Rasmussen told reporters.

NATO has made clear it does not want to send troops to protect Ukraine, which is not a member country.

Rasmussen’s remarks came amid renewed fighting in eastern Ukraine, where government troops are mounting an offensive against the rebels who have Russian citizens among their ranks. Among them is Alexander Borodai, a Muscovite who resigned Thursday as prime minister of the self-proclaimed Donetsk People’s Republic, saying it should be led by a Ukrainian.

In Donetsk, mortar shells struck three apartment buildings and a large hospital near the city center. The dentistry wing of Vishnevskiy Hospital was damaged, and one person died. Many residents of the east say the Ukrainian military is responsible for the shelling. The military insists its artillery is directed only at convoys bringing weapons and equipment to the rebels. It says it does not shell or bomb cities, blaming that on insurgents trying to discredit the military.

In an odd incident that underscores the hybrid nature of the conflict, Ukraine accused Russia of holding dozens of military officers and border guards against their will. The officers and guards were among hundreds who fled to Russia Monday to escape heavy bombardment from the rebels who are supported by Russia.

While most have returned to Ukraine, military spokesman Andriy Lysenko said 18 officers and 28 border guards were not allowed to leave. Five of the officers, including two commanding officers, have been arrested and charged with directing artillery attacks into Russian territory, he said. They have faced interrogations that Lysenko characterized as coercive.

Lysenko also repeated a theory about the downing of the Malaysia Airlines passenger plane, first espoused earlier Thursday by Valentyn Nalyvaichenko, the head of Ukraine’s Security Service. The plane was shot down July 17 over separatist territory by what the U.S. and Ukraine say was a missile supplied to rebels from Russia.

Nalyvaichenko said his agency had received “information” that separatists intended to shoot down a Russian Aeroflot plane flying from Moscow to Larnaca in Cyprus, causing an uproar that would induce Russia to invade Ukraine.

He said that because the rebels were Russians, not local, they became confused and ended up driving to the village of Pervomaisky in rebel-held territory instead of another village of the same name that is in Ukrainian-held territory. That mistakenly brought them under the flight path of the Malaysian jet, not the Aeroflot plane, he said

Meanwhile, protesters in Kiev clashed Thursday with municipal workers trying to clear away the debris that has festered for months in the city’s heart, Independence Square. The area, known as the Maidan, was the center of an uprising that eventually ousted the pro-Russian president, Viktor Yanukovych.

Many residents say the Maidan encampment has outlived its purpose and want it dismantled. But a small corps of people remain there, saying they are keeping the movement’s spirit alive.