Ukraine's pro-Russian rebels rushed Sunday to dispel speculation that a bloody internal battle for control of Moscow's money was behind the gangland-style slaying of a top warlord.

Prizrak (Ghost) brigade commander Alexei Mozgovoi and at least six others -- including his press secretary and armed guards -- were shot dead Saturday while traveling in the renegade eastern province of Lugansk, a region under effective rebel control.

A separatist commander in eastern Ukraine was killed along with several other fighters after their car came under attack Saturday, rebel authorities announced.

The defense ministry of the self-proclaimed Lugansk republic confirmed that Alexey Mozgovoy, the commander of a police battalion in the war-torn region, was among the dead and said it was hunting for the assailants behind the attack.

A top pro-Russian rebel said on Friday that a new round of Ukraine crisis talks had resulted in no breakthrough but would continue in the coming weeks.

The chief negotiator of the self-declared Lugansk People's Republic said the European-mediated talks got stuck on the crucial issue of when and under what terms Ukraine's two renegade eastern provinces could conduct their own elections.

President Vladimir Putin's spokesman said Friday the Kremlin was doing all it could to secure the release of two men that Ukraine says are captured Russian soldiers.

"These are Russian citizens who are in captivity," Putin's spokesman Dmitry Peskov told journalists, quoted by TASS state news agency.

Amnesty International on Friday accused Ukrainian forces of torture and pro-Russian rebels of even more serious war crimes such as summary executions committed both before and after a February truce deal.

The damning report adds to a growing string of independent findings suggesting that Europe is witnessing the most brutal conflict on its outer border since the 1990s Balkans crises.

Two Russians captured by Ukrainian forces during a firefight in the ex-Soviet state's separatist east have admitted to serving in the Kremlin's armed forces, the OSCE said on Thursday.

"Both individuals claimed that they were members of a unit of the Armed Forces of the Russian Federation. They claimed that they were on a reconnaissance mission. They were armed but had no orders to attack," the Organization for Security and Co-operation in Europe reported after conducting interviews with the two wounded men in a Kiev hospital.

German Chancellor Angela Merkel said Thursday a return of Russia to the Group of Seven major industrialized nations is "unimaginable" as long as it flouts international law in Ukraine.

"As long as Russia does not commit itself, and act according to, the fundamental values of international law, a return to the G8 format is unimaginable for us," Merkel, who hosts a G7 summit next month, told parliament.

Ukraine has charged two suspected Russian soldiers who were captured during a gun battle in the separatist eastern region of Lugansk with involvement in "terrorist activity", a security official said Wednesday.

Ukrainian Security Service spokesman Markiyan Lubkivsky wrote on Facebook that Captain Yevgeny Yerofeyev and Sergeant Aleksander Aleksandrov, now convalescing in a Kiev military hospital, were also allowed to telephone their relatives in Russia.

Ukrainian President Petro Poroshenko sparked fresh Kremlin fury on Wednesday by warning that his crisis-torn country was fighting a "real war" against Russian aggressors that could escalate at any time.

The pro-Western leader said the weekend capture of two purported Russian special forces members proved that the separatist uprising in the industrial east of Ukraine was a guise for a Moscow-orchestrated campaign aimed at breaking up the ex-Soviet state.

NATO head Jens Stoltenberg urged Russia on Tuesday for full transparency during snap military exercises in order to avoid potentially catastrophic misunderstandings as the Ukraine crisis stokes tensions.

Transparency is paramount, Stoltenberg said after talks with Russian Foreign Minister Sergei Lavrov on the sidelines of a Council of Europe meeting in Brussels.