The Organisation for Security and Cooperation in Europe (OSCE) has said that pro-Russian rebels in eastern Ukraine have released four of the group's monitors they had detained at the end of May.

"Our Lugansk-based team of four monitors have been released after one month in captivity. Details to follow," the OSCE Special Monitoring Mission to Ukraine said on its Facebook page.

Two groups of unarmed observers from the Vienna-based European security organisation - totalling eight international monitors and a Ukrainian translator - were detained at roadblocks three days apart in late May.

Separatist leaders on Thursday freed the group they had detained on 26 May in the Donetsk region.

But the second team seized on 29 May in the neighbouring Lugansk province appeared to have been abducted by a different militia and negotiations for their release intensified in the recent days.

A Donetsk rebel spokeswoman told AFP that the second team would be presented to reporters at a local hotel later today.

Russian President Vladimir Putin publically called on the pro-Kremlin militia to free all people held against their will under the terms of a temporary ceasefire in the nearly three-month insurgency agreed with Kiev at the end of last week.

Ukraine extends ceasefire until Monday night

Earlier, Ukraine extended a government forces' ceasefire against separatist rebels by 72 hours until 10pm on Monday.

The announcement came shortly after President Petro Poroshenko returned to Kiev from Brussels, where he signed a landmark free-trade deal at a European Union summit.

The ceasefire extension had been undertaken, Mr Poroshenko’s website said, in line with a deadline set by EU leaders for Ukrainian rebels to agree to ceasefire verification arrangements, return border checkpoints to Kiev authorities and free hostages including the detained monitors.

At a separate meeting, Mr Poroshenko and national security chiefs said that during the next 72 hours recruitment centres for Russian fighters across the border in Russia should be closed.

Movements of rebel forces around the east and the setting-up of rebel checkpoints or barricades should also cease.

Ukrainian government forces would have the right to end the ceasefire ahead of time in any areas where ceasefire conditions were not being implemented, the announcement said.