Russian energy company Gazprom in June cut gas supplies to Ukraine because of lingering debt disputes. The Ukrainian government said testing began to send 70 million cubic feet per day from Slovakia to Ukraine through the joint work of transit companies Uktransgaz and Eustream.
Commercial operations from the Slovakian transit system could start as early as Sept. 2.
Uktransgaz Chairman Igor Prokopiv credited his company's professionalism with keeping energy issues in Ukraine in check.
"Ukrtransgaz will have a lot of strategic objectives ahead, but I am convinced ... we will be able to comply with them," he said in a statement.
German energy company RWE in April said it reversed the flow of natural gas from Poland to supply a Ukrainian market under the Kremlin's influence.
RWE says up to 353 billion cubic feet of natural gas per year would flow from Poland to Ukraine under the terms of a contract signed in 2012.
Ukraine's pivot toward the European Union in November sparked a geopolitical tug-of-war between Western powers and the Kremlin. That geopolitical row led to heightened concerns over European energy security as most of the gas sent by Russia to EU members runs through Ukraine's Soviet-era pipeline network.