KIEV: Just three days after Ukrainian soldier Oleksandr Lazarenko was taken prisoner in the war in Ukraine's east, a peace agreement was signed under which the sides agreed to an "all-for-all" prisoner exchange. A year and a half later, his wife still waits desperately for his return.

The so-called Minsk Agreement on ending the war between Russia-backed separatists and Ukrainian forces is floundering on many issues, but prisoner releases appear to be one of the most intractable.

Amnesty International and Human Rights watch allege that both sides have arbitrarily detained civilians, sometimes holding them incommunicado for months in prisons that authorities don't acknowledge exist.

The sides cite widely varying figures for how many prisoners they're holding. Both appear to be holding the issue over the other's head to force concessions on other topics, which include holding elections in Ukraine's separatist region and restoring Ukraine's full control over its border with Russia.

"Our expectations have been thwarted by politicians' plans," Lazarenko's wife, Natalya, told The Associated Press. "After Minsk, the prisoners became an instrument of political trading, they're seen as political commodities."

Even that frustration and anxiety is something of an improvement. For three months after her husband was seized, she had no information on whether he was dead or alive. Lazarenko was captured by a Cossack formation that was not under the rebels' control. Eventually, he and 12 others were found by the rebel government's Committee on POWs and transferred to a detention facility in the eastern Ukrainian city of Donetsk, the rebels' main stronghold.

Finally allowed to call his wife, Lazarenko said the Cossacks had held him in a windowless basement where he could only lie down on sacks of potatoes. He told her that he and the other prisoners were beaten and fed scraps.

"Sasha simply ceased to exist," his wife said.

Although the numbers held by each side are in dispute, it's clear that the pace of releases has slowed markedly for Ukrainians held by the rebels. Since the Minsk agreement was signed in February 2015, 83 Ukrainian prisoners have been released, but only 12 of them were freed this year.

Before the agreement, prisoners were handed over more freely. Yuri Tandyt, an adviser to Ukraine's national security service, told Ukrainian media in August that a total of 3,080 Ukrainian prisoners had been released by rebels or had been located since the beginning of fighting in April 2014. Ukraine now lists 112 names as soldiers held by the rebels, suggesting that thousands had been released prior to the Minsk agreement.

The separatists acknowledge only holding less than half that many. Irina Gerashchenko, Ukraine's deputy parliament speaker and a key figure in negotiations to implement the Minsk agreement, says the rebels admit to holding 47 and "we don't know where the other hostages are held."

Rebel officials said last month they had reached a tentative agreement with Ukraine to release 47 prisoners, in exchange for Ukraine freeing 618.

Who those 618 might be is uncertain. Rebel military spokesman Eduard Basurin told the AP that Ukraine is holding 962 easterners, of whom 316 are fighters and the rest are either political prisoners or civilians with no connection to the conflict. Ukraine in turn says it is holding about 500 people in connection with the war.

Vadim Karasev, a Ukrainian political analyst, suggests that many of those held by the Ukrainian side are not combatants.

"Kiev is seeking to increase its weight in the negotiations by the count of separatists detained," he said. "Simply disgruntled citizens often end up (in that category), and then are proposed for exchange."

"In many cases, the only reason for the detention of prisoners may be to use them as bargaining chips," said Oksana Pokalchuk, Amnesty International's executive director for Ukraine.

The Organization for Security and Cooperation in Europe, the mediator in the peace process, "does not have the instruments to force the exchange of prisoners. Kiev and the separatists cannot even agree on the prisoner lists. The OSCE cannot do this work for them," Karasev said.

Nikolai Vakaruk, a 34-year-old coal miner, says he was a blameless civilian improperly held in detention by Ukraine for a year and half.

He told the AP he was seized during a search of his home in the front-line town of Ukrainsk and held in the security-service detention facility in Kharkiv, where he was repeatedly beaten in an attempt to get him to confess to being a separatist.

"I was beaten and tortured but they could not turn me into a separatist," said Vakaruk, who believes he was detained for being a critic of the Ukrainian authorities. Amnesty International says Vakaruk was one of 13 prisoners released from the Kharkiv facility in July following the group's report on war prisoners.

Vakaruk also said when international groups came to Ukraine's security services building, he and other prisoners held in connection with the war were spirited away to other locations.

"I realized that in the new Ukraine, I can disappear just because I think differently," he said.