Russia’s state-controlled Gazprom is warning it could limit natural gas supplies to European customers if they to try to sell any of it back to Ukraine.

Kyiv wants large-scale so-called reverse flows of gas, for example via Slovakia through a old, currently unused, pipeline. Poland is another possible route.

Gazprom CEO Alexey Miller said they are monitoring the situation closely, and he virtually accused Ukraine of syphoning off gas in transit to Europe.

After Gazprom’s annual shareholders’ meeting, Miller told a news conference: “Our Russian gas is now staying in Ukraine, Ukraine is using our Russian gas as its own. Right now there’s no physical reverse flow of gas. I think there could come a time in the near future when we will probably introduce limitations of deliveries to [the European] gas measuring stations, where we will be able to see if there is a reverse flow.”

Miller described any reverse flows of gas as a “half-fraudulent scheme”.

He would not say which European countries might have their supplies limited.

Gazprom cut off gas to Ukraine on June 16 following Kyiv’s failure to pay off some of the billions Moscow says it owes in gas debts.

Gazprom meets around a third of Europe’s gas import needs, while half of Russian gas exports to Europe go via Ukraine.

It used to be around three-quarters, but the share has declined after it launched the Nord Stream pipeline with an annual capacity of 55 billion cubic metres (bcm) via the Baltic Sea directly to Germany in 2011.

Gazprom now plans to build the South Stream pipeline to ship gas to Europe across the Black Sea in 2015 with a view to increasing its annual capacity to 63 bcm by 2018-2019 despite opposition from the EU, which says the project does not meet its regulations.

“After we build South Stream, taking into account Nord Stream and other routes, there will be almost no need left to use Ukraine’s gas pipeline system,” Gazprom’s Deputy Chairman Alexander Medvedev said.