Kiev on Thursday said four soldiers had been killed and 14 injured in renewed clashes with pro-Russian insurgents in the former Soviet country's separatist east.

Military spokesman Oleksandr Motuzyanyk told reporters that the losses came in Lugansk -- the smaller of the two separatist provinces in Ukraine to have fallen under partial militia control in the past 16 months of conflict.

The United States on Wednesday called on Russia to release an Estonian intelligence officer who was sentenced to 15 years in prison for spying and weapons possession, calling the charges "baseless."

The sentencing of Eston Kohver, an officer for Estonia's KAPO internal security service, came in a court in Russia's western Pskov region, which borders Estonia.

Russian President Vladimir Putin would be open to meeting with American counterpart Barack Obama on the sidelines of the U.N. General Assembly in New York, Russia's foreign minister said Wednesday. 

"Our American colleagues are sending us signs that they want to maintain contact," Sergei Lavrov said, after confirming that Putin was planning to attend the United Nations meet new month.

Germany said Wednesday it could not rule out Russian President Vladimir Putin would take part in a future round of talks over conflict-torn eastern Ukraine after speculation that Western leaders were snubbing him.

Chancellor Angela Merkel is hosting talks with France's President Francois Hollande and Ukrainian leader Petro Poroshenko on Monday, and analysts said the conspicuous absence of Putin underlined that relations were deteriorating as new violence flares in eastern Ukraine.

The daughter of slain Russian opposition politician Boris Netmsov has filed a complaint over investigators' refusal to question Chechen leader Ramzan Kadyrov over her father's murder, her lawyer said on Wednesday.

Nemtsov, a 55-year-old former deputy prime minister who had become a vocal critic of President Vladimir Putin, was gunned down on a bridge just steps from the Kremlin in late February.

The OSCE on Wednesday accused rebels controlling parts of east Ukraine of barring access to their monitors, as fears grew that escalating violence could flare up into a return of all-out war.

"They deny us access to specific areas. We are not allowed through at checkpoints," Alexander Hug, the deputy chief of the OSCE's monitoring mission in eastern Ukraine, told Germany's Die Welt newspaper.

Russia on Wednesday sentenced an Estonian intelligence officer to 15 years in jail, drawing ire from the EU after Tallinn said he was kidnapped at gunpoint on its territory.

A court in the western Pskov region, which borders Estonia, sentenced Eston Kohver on charges of spying, weapon possession and illegally crossing the border, his court-appointed lawyer, Yevgeny Aksyonov, told AFP.

Ukraine's pro-Western leadership and Russia traded blame on Tuesday for a sharp escalation in violence that threatened to see all-out warfare return to the former Soviet nation's separatist east.

Kiev and the insurgents on Monday reported the deaths of at lest 10 soldiers and civilians -- a dramatic escalation that sparked international condemnation and marked the worst bloodshed in more than a month.

U.N. Secretary-General Ban Ki-moon on Monday expressed serious alarm over the upsurge in fighting in Ukraine and called for urgent steps to de-escalate the violence.

Fierce clashes in east Ukraine killed at least 10 people in the past 24 hours, government and rebel officials said Monday, as Russia warned that Kiev could be preparing for a fresh offensive.

Russian President Vladimir Putin on Monday slammed alleged "external control" over Ukraine's government as he made his third visit to the Crimean peninsula since Moscow seized the region from Kiev last year.

"I am sure that despite all the current difficulties the situation in Ukraine will improve and Ukraine will develop," Putin said in televised comments during a meeting with local officials.