The self-styled mayor of the flashpoint city of Slavyansk, Vyacheslav Ponomaryov, told reporters: “There has been good progress and a good productive dialogue.”

Ponomaryov’s comments offered hope that the monitors — four from Germany, one Dane, one Pole and one Czech — might soon be released after five days in captivity.

Earlier Tuesday, the head of the Organisation for Security and Cooperation in Europe, Lamberto Zannier, held emergency talks in Kiev with the US ambassador to Ukraine and the foreign minister.

The minister, Andriy Deshchytsia, demanded the “immediate release” of the men, whose detention sparked international outrage and raised the stakes in the worst East-West crisis since the Cold War.

The US ambassador, Geoffrey Pyatt, stressed that the men should be released “without conditions”.

Rebels holding the group have previously described them as “prisoners of war” and said they will only be freed in exchange for the release of militants in custody. More recently they have taken to calling them “guests”.

They are also holding four Ukrainians linked to the OSCE mission, whose fate is unclear.

Eight of the European monitors appeared before the cameras on Sunday and one of them said they were in good health.

One of the men, a Swede, was later freed on medical grounds as he suffers from diabetes.

Ponomaryov told reporters that this was a “goodwill gesture” made by the separatists.

As international efforts intensified to secure the release of the monitors, the Austrian foreign minister and the head of the Council of Europe were due in Kiev on Wednesday to push negotiations forward.

The OSCE is in Ukraine to assess progress towards an April 17 accord struck in Geneva that was supposed to defuse tensions.

Part of that accord was that pro-Russian militants should leave occupied buildings but this has not happened, with the West accusing Moscow of failing to exert its influence over them.

burs-ric/rmb/cc