Ukraine's largest rebel-controlled city of Donetsk was shaken by violence Wednesday as 10 people were killed, some of them teachers and parents showing up for the first day of school.

Terrified children were forced to shelter in the basement as the shelling yielded the highest daily civilian toll since a tenuous ceasefire was struck between Kiev and pro-Russian separatists last month.

The European Union on Tuesday decided to keep in place biting economic sanctions on Russia over its alleged backing of pro-Kremlin insurgents who have largely ignored a four-week Ukrainian truce.

The bloc's announcement came just moments before NATO reported the continued presence of "hundreds" of elite Russian soldiers in the war-torn former Soviet nation's separatist east.

Ukraine's tenuous truce and troop withdrawal deal lay in tatters on Tuesday after the deadliest wave of attacks by pro-Russian insurgents in more than a month killed nine government soldiers.

The surge in clashes across the separatist rust belt spelled an ominous start to campaigning for parties that make the ballot for October 26 parliamentary polls once the registration deadline passes on Tuesday night.

With the world on high alert over foreign fighters joining jihadist ranks in Syria and Iraq, Balkan states are launching efforts to clamp down on recruiting in their region, considered fertile ground by Islamists.

Of the more than 20 million people in southeast Europe, more than five million are Muslims, and an economic slump in weak states battered by past wars has fired up some of the disenfranchised.

Students returning for the new school year in eastern Ukraine on Wednesday will get real-life conflict lessons as they take their seats in battle-scarred classrooms.

At one high school in rebel-held Donetsk, teachers, students and parents are refusing to allow the conflict to derail studies, joining forces to clear away rubble, replace blown-out windows and set buckets to catch leaks, a month after four rockets destroyed the roof.

A Russian court ruled on Monday that a Ukrainian film director charged with terrorism must stay in jail until January despite international appeals for his release.

Oleg Sentsov was arrested in May in Simferopol, Crimea's main city, by Russia's FSB security service and charged with plotting and carrying out "terrorist attacks" after the region was annexed by Russia.

Thirteen soldiers and civilians have died in Ukraine, officials said Monday, in the deadliest surge of violence since the government and pro-Russian insurgents struck a shaky truce in their five-month conflict.

Nearly a month after tense talks yielded a peace pact and a ceasefire agreement envisioning a buffer zone on the front line, a lasting solution to the insurgency, which has killed more than 3,200 people, seemed no closer.

Ukraine held back to see whether a Russian-backed truce would hold through the day before beginning its own withdrawal of forces from the frontline stretching across the war-torn country's separatist east.

The military reported suffering no casualties but more than a dozen overnight attacks overnight by pro-Russian insurgents who refuse to accept the new Kiev leaders' decision to seek a closer alliance with the West.

Russian Foreign Minister Sergei Lavrov thinks relations between Moscow and Washington need a new "reset", he says in an interview to be aired Sunday, blaming the freeze on the United States.

"Now what's needed is something that the Americans will call a 'reset'," Lavrov told Russia's Channel 5 television, according to a transcript published on the foreign ministry's website.

Former Norwegian premier Jens Stoltenberg will on Wednesday take charge of a revitalized NATO which less than a year ago looked like a Cold War dinosaur in a fast-changing world.

The alliance has a new-found sense of purpose thanks to the Ukraine crisis but Stoltenberg will be aware that it must also face up to many other and longer-term challenges, analysts said.