The Organization for Security and Cooperation in Europe said on Friday it had lost contact with a second four-member international team in restive eastern Ukraine.

The Vienna-based security body said it had not heard from its team in the industrial region of Lugansk since Thursday evening when it was stopped "by armed men" at a roadblock in the town of Severodonetsk.

Ukraine claimed it had regained control of swathes of the separatist east on Friday even as Washington expressed concern over the appearance of fighters from Russia's war-ravaged region of Chechnya among the insurgents.

The rebels for their part dismissed speculation of a rift in their ranks after a dozen local militants were evicted from their seat of power in Donetsk by a military brigade comprised largely of Chechens and other Russians from the volatile North Caucasus.

Russian troops massed on Ukraine's borders are moving back toward Moscow, but there are still "danger signs," U.S. Secretary of State John Kerry said late Thursday.

"There is evidence of Russians crossing over, trained personnel from Chechnya trained in Russia, who've come across to stir things up, to engage in fighting," the top U.S. diplomat told PBS television.

Russia on Thursday called on the West to prevent “a national disaster” in Ukraine as further deadly violence erupted in the country's east, killing 12 soldiers.

"We once again call on our Western partners to use all their influence on Kiev to stop Ukraine from descending into a national disaster," the ministry said in a statement.

Pro-Russian rebels downed a Ukrainian helicopter Thursday, killing 12 soldiers including a general and undermining president-elect Petro Poroshenko's fervent vow to crush the bloody seven-week insurgency roiling the industrial east.

The militants' success was followed by the surprise admission by one of their leaders that 33 of more than 40 rebels killed during a raid on a Donetsk airport at the start of the week were Russian nationals from Muslim regions such as Chechnya.

Separatist rebels in eastern Ukraine said Thursday they were holding four unarmed European monitors who went missing three days earlier but promised to release them soon.

The latest abduction in a vital rust belt region overrun by pro-Russian militants since early April underscores the trouble newly-elected president Petro Poroshenko will have in keeping his ex-Soviet republic whole.

Ukraine's prime minister said on Wednesday that Europe would never again have a "Berlin Wall" and that his country wanted to become European, ahead of talks with German Chancellor Angela Merkel.

Arseniy Yatsenyuk said the Ukrainian people wanted a "European perspective" for the strife-torn former Soviet republic, which is facing an armed secessionist movement in parts of the east.

The president-elect of Ukraine told a German newspaper Wednesday that he planned to hold talks with Russian President Vladimir Putin in a bid to "ease" the crisis in Ukraine.

"We will hold talks with Putin in order to ease the situation and make peace. When and where these talks will take place, is not yet decided," Petro Poroshenko told the Bild daily.

Russia on Wednesday said there was no justification to Ukraine's offensive against eastern separatists and demanded "urgent steps" be taken to end the seven-week campaign.

Russian Foreign Minister Sergei Lavrov told his German counterpart Frank-Walter Steinmeier by telephone that "there can be no justification for the punitive operation" and called for "urgent steps to be taken to end the bloodshed and to launch a dialogue between the Ukrainian people, including on the subject of sweeping constitutional reform".

The German government called on Russia Wednesday to step up its border controls to prevent pro-Russian separatists or weapons going into crisis-hit Ukraine.

Berlin expects that Moscow "along its border with Ukraine prevents separatists ready for violence from infiltrating, or that large quantities of weapons are transported", spokesman Steffen Seibert told reporters.