Two groups of observers from the Organisation for Security and Cooperation in Europe (OSCE) -- totalling eight international monitors and a Ukrainian interpreter -- have been held captive and incommunicado since separatists picked them up, three days apart, in late May.
The abductions came after seven OSCE observers sent to Ukraine by the Vienna-based organisation were held for eight days by pro-Russian rebels in their stronghold city of Slavyansk and released in early May.
He was speaking after attending a two-day OSCE conference hosted by Japan, which discussed the situation in Ukraine, where militants continue to defy the government in Kiev.
While some rebel leaders in Slavyansk have reportedly claimed the observers are in their custody, others say they have no idea where they are.
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Cases of arbitrary detention and abductions are on the rise in the areas of eastern Ukraine controlled by pro-Russian rebels, rights activists and the UN have said.
"That is what we've been encouraging... The Russian government...To take actions to secure border to make sure that arms and materials and people aren't crossing the border to contribute to the violence in Ukraine," he said.