Pro-Russian insurgents launched a fresh assault Thursday on an airport held by isolated Ukrainian forces as a month-old truce came under renewed strain and calls grew for the Kremlin to help halt the bloody revolt.

Ukraine also dispatched its energy chiefs to Brussels in a bid to convince the European Union to back up Kiev at crunch talks with Russia on the latest energy war with its westward-leaning but effectively bankrupt neighbor.

New NATO chief Jens Stoltenberg said Wednesday Russia must reverse course in Ukraine but stressed the alliance still remains ready to have a constructive relationship with Moscow.

Russia's intervention in Ukraine "is a major challenge to Euro-Atlantic security", former Norway prime minister Stoltenberg told his first press conference as NATO head after replacing Anders Fogh Rasmussen.

Russia on Wednesday vowed to protect its citizens in the Moldovan region of Transdniestr, warning "those who don't think like us" not to meddle in the pro-Moscow breakaway region.

"There is no need to indulge in any illusions about whether Russia will protect its own citizens," the state news agency TASS quoted Deputy Prime Minister Dmitry Rogozin as saying.

He has been tormented by a bloody pro-Russian uprising and the near-bankruptcy of his young but bitterly conflicted former Soviet state.

Now President Petro Poroshenko's success in handling both crises will be tested when Ukrainians pick a new parliament that could make or break his post-war revival plans.

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.

A high-ranking military delegation kicked off a visit to Moscow to discuss with Russian officials the needs of the army and security forces, al-Joumhouria newspaper reported on Wednesday.

The talks between the Lebanese army officials and their Russian counterparts is expected to pave way for a deal with Rosoboronexport, the sole state intermediary agency for Russia's exports/imports of defense-related products, the daily said.

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.

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.

Poland on Monday said a possible French deal to supply Russia with warships was making it difficult for Warsaw to choose French suppliers for its planned missile shield.

"I can't hide the fact that the Mistral (warship contract) is not helping us make positive decisions" about French missile shield suppliers, Polish Defense Minister Tomasz Siemoniak said in an interview published in the Rzeczpospolita daily.