Ukraine rebels 'to free' observers
Pro-Russia separatists in eastern Ukraine have admitted they are holding four international monitors and said they may free them soon, Interfax says.
"We'll clarify who they are, where they were heading and why, and will set them free," the self-proclaimed mayor of Sloviansk told the Russian news agency.
The members of the Organization for Security and Co-operation in Europe were stopped near Donetsk on Monday.
Fighting between pro-Kiev and rebel forces has worsened in recent days.
The four missing monitors - who are Turkish, Swiss, Estonian and Danish - are members of a monitoring mission sent to assess security in the area.
The town's self-proclaimed Mayor Vyacheslav Ponomaryov told Interfax on Thursday: "We know where they are, all is well with them."
He said the men had decided to travel to a separatist-held area despite having been warned "against going anywhere for a while" - the four held had been "the most eager ones".
"Of course they were detained," he said, although he insisted they had not been arrested.
The OSCE has said it does not know the monitors' whereabouts, but Mr Ponomaryov told another Russian news agency they were being held in the village of Makeyevka.
The BBC's Mark Lowen in eastern Ukraine says there have been cases of kidnapping as lawlessness has reigned amid fighting between Ukrainian forces and pro-Russia militias.
People have been either detained or attacked at rebel checkpoints.
Ukraine's President-elect Petro Poroshenko has vowed to tackle the eastern uprising, promising to deal firmly with "bandits" and "murderers".
On Wednesday the OSCE said it had lost contact with another 11 observers near Donetsk, but the group got back in touch later in the day.
Last month the rebels held seven OSCE-linked military observers for a week in Sloviansk, before releasing them without condition.
Also in April, US journalist Simon Ostrovsky was abducted near Sloviansk, saying afterwards he had been beaten and held in a basement for three days.
Mr Poroshenko, a confectionery magnate, won 54.7% of the vote in last Sunday's presidential election, according to final results announced on Thursday,
Voting was severely disrupted in the east, which has also seen an upsurge in fighting this week.
The separatists say they lost up to 100 fighters as they tried to seize Donetsk airport from pro-Kiev forces on Monday.
Further clashes were reported in several areas in recent days.
Pro-Russian separatists in the regions of Donetsk and Luhansk declared independence after referendums on 11 May, which were not recognised by Kiev or its Western allies.
The separatists took their cue from a disputed referendum in Crimea, which led to Russia's annexation of the southern peninsula.
Kiev and the West accuse Russia of stoking separatist sentiment in eastern Ukraine - a claim President Vladimir Putin denies.
Meanwhile Russian state-run energy giant Gazprom has told Ukraine it has until midnight on Thursday to pay $2bn (£1.2bn; 1.4bn euros) it says it is owed by Kiev.
If no payment is made, Gazprom has threatened a halt in gas supplies next week which would affect parts of Europe.