Instead, the majority state-owned Slovak company that runs the system, Eustream, has offered only a small, long-disused subsidiary pipeline that still needs engineering work before it can carry gas to Ukraine. Once the work is finished in October, Eustream will provide just a tenth of the gas Ukraine has been looking for from Europe. The company says that small amount can be increased sharply later.

Here in Chaslovtsy, in southwestern Ukraine, where technicians from Ukraine’s pipeline company, Ukrtransgaz, and Gazprom monitor the flow of Russian gas into Slovakia, the Ukrainian head of the facility, Vitaly Lukita, said he wondered if gas would ever flow the other way.

“We are all ready here, but I don’t know why the Slovaks are taking so long,” Mr. Lukita said. “Everyone has been talking about this for a very long time, but nothing has happened.”

Andriy Kobolev, the board chairman of Naftogaz, Ukraine’s state gas company, said he was particularly mystified by the recalcitrance of Eustream because in 2011 the company had put forward the idea of using spare capacity in its trunk pipelines for reverse-flow supplies to Ukraine.

He said the Slovaks had rejected this option in recent negotiations, citing secret contracts with Gazprom. He added that he did not know what the problem was exactly, because he had not been allowed to see the contracts.

Eustream executives declined repeated requests for interviews. Vahram Chuguryan, the company’s spokesman, declined to comment on the apparent change of heart or on whether it was related to an ownership shuffle in early 2013, when a group of wealthy Czech and Slovak businesspeople purchased a 49 percent stake in Eustream. At the time, Czech news media speculated that they were acting as a stalking horse for Gazprom.

Daniel Castvaj, a spokesman for Energeticky a Prumyslovy Holding, the company that made the purchase, denied Ukrainian assertions that Eustream has sought to limit reverse-flow deliveries to Ukraine, describing these as “not only untrue but nonsensical” since the pipeline operator, which makes its money off transit fees, has a strong commercial interest in boosting flows regardless of direction.