Ukrainian forces and pro-Kremlin insurgents said Friday they have swapped dozens of prisoners under the terms of a fragile truce aimed at ending the five-month eastern revolt.

Separatist leader Andrei Purgin said 31 guerrillas were swapped overnight for 36 Ukrainian soldiers in a town north of the rebel stronghold of Donetsk.

Nordic and Baltic foreign ministers on Friday demanded Russia immediately free an Estonian policeman charged with espionage in a case that has heightened tensions between Estonia and its Soviet-era master. 

Russia on Friday slammed the European Union for adopting new sanctions against Moscow during a week-old ceasefire agreed for conflict-riven eastern Ukraine.

"We believe that adopting such decisions at the very moment when the peace process in Ukraine is gaining strength -- we are hoping -- ... this means choosing a path towards undermining the peace process," Russian Foreign Minister Sergei Lavrov said in televised remarks on the sidelines of a security summit in the Tajik capital Dushanbe.

Chinese President Xi Jinping called for Beijing and Moscow to offer each other a "helping hand" in the face of external challenges, at talks with his counterpart Vladimir Putin in Tajikistan Thursday, state media reported.

Washington's concerns that Russia violated a 1987 arms control treaty "were not assuaged" during a meeting between the two sides in Moscow, the State Department said Thursday.

U.S. negotiators nevertheless felt they had a "useful exchange of views" with their Russian counterparts and have "agreed to continue the dialogue," it said in a statement.

A senior Ukrainian defense official said on Thursday there was no contradiction between President Petro Poroshenko's announcement of a major Russian troop withdrawal and NATO's assertion that 1,000 Kremlin troops remained in the eastern war zone.

The high-ranking official said Ukrainian military assessments had shown up to 5,000 Russian soldiers crossing the border to bolster the pro-Kremlin uprising that began in mid-April and escalated in recent months.

U.S. air strikes on Syrian territory without permission from the government in Damascus would be an "attack" on the country, a Syrian minister said Thursday.

"Any action of any kind without the consent of the Syrian government would be an attack on Syria," National Reconciliation Minister Ali Haidar said when asked about U.S. plans to widen its operations against Islamic State (IS) jihadists with air strikes on Syrian territory.

U.S. President Barack Obama said Thursday that the United States would intensify sanctions in the defense, finance and energy sectors to punish Russia over its "illegal actions" in Ukraine, as the European Union said it could lift a new set of sanctions if a Ukraine ceasefire holds.

Obama said in a statement the move would worsen the Kremlin's deepening political and economic isolation.

For the Ukrainians who have fled the war zones of the east, life has become a grim daily grind of battling red tape and trying to find food, jobs and shelter.

The European Union failed to reach agreement on when the latest economic sanctions against Russia should be implemented and will meet again Thursday for more talks, a diplomatic source said.

The 28 member state ambassadors to the EU will "continue today's discussion on the restrictive measures, allowing for further assessment of the situation on the ground and additional consultations," the source said.