A local staffer for European monitors observing Ukraine's flaring separatist conflict was returned Wednesday, a day after going missing in the country's pro-Russian east.

The Organization for Security and Cooperation in Europe (OSCE) said a member of its Special Monitoring Mission (SMM) did not return from a planned vacation on Tuesday and was reportedly being held in the insurgents' de facto capital of Donetsk.

The employee was returned only on Wednesday evening, after being gone for about a day, and after the OSCE mission demanded his "immediate and unconditional release."

"The SMM staff member who went missing on 7 June was returned today to the mission in Donetsk," the mission said on its Facebook page.

A mission spokeswoman contacted by AFP declined to give immediate details on the matter.

A source told AFP on condition of anonymity that the missing Ukrainian was a driver who lived in the neighbouring pro-Russian region of Lugansk.

The Donetsk rebels' human rights representative called the OSCE's report "a fake".

"We did not detain anyone," Darya Morozova told AFP by phone. "We are not holding any OSCE staff."

The incident is the latest in a string of cases of OSCE and other foreign monitors going missing in the war-scarred industrial southeast of the former Soviet state.

Pro-Russian fighters usually release their captives after protracted negotiations that can take weeks or even months.

But the United Nations' human rights representative Ivan Simonovic said last week that he still had no information about the fate of a staff member reported captured in Donetsk on April 13.

- Fighting 'is worsening' -

Ukraine's 26-month war has claimed the lives of nearly 9,400 people and recently the violence has returned to levels not witnessed since the signing of a December 2015 truce.

Kiev and its Western allies accuse Russia of plotting and directly supporting the rebellion in the east, in retaliation for Ukraine's February 2014 ouster of its Moscow-backed leadership.

Russia denies the charges but backs the insurgents' demands at international venues including the UN.

The deputy head of the OSCE's monitoring mission warned on Wednesday that the situation in the war zone "is worsening".

"We rightly concentrate on the failure of the sides to withdraw weapons from the security zone, the continued use of those weapons and the failure to separate the sides, and in some cases positioning even closer to one another," Alexander Hug told reporters.

Hug added that most of the ceasefire violations were being recorded around the ruins of an airport just north of Donetsk that the militias captured in January 2015 at a heavy cost to both sides.

- 'Shattered illusions' -

A repeatedly-broken truce deal brokered by Germany and France in February 2015 required both sides to withdraw their weapons from a 30-kilometre (19-mile) wide buffer zone splitting rebel-run regions from the rest of Ukraine.

The conflict has wrecked Moscow's relations with Washington and seen Russia respond to Western economic sanctions by banning the import of most Western food.

The US State Department's chief envoy to Europe told the Senate Foreign Relations Committee on Tuesday that the Kremlin was refusing to abide by international law and forcing Washington to re-evaluate its position toward Moscow.

"Russia's invasion of sovereign Ukrainian territory... shattered any remaining illusions about this Kremlin's willingness to abide by international law or live by the rules of the institutions that Russia joined at the end of the Cold War," Victoria Nuland said.

Moscow accuses Washington of inciting three months of protests that led to the Moscow-backed administration's downfall and Kiev's subsequent turn toward the West.

Source: AFP