Ukraine pumped around 600 million cubic metres into underground gas in July, the Association of European Underground Gas Storage Operators said.
Supplies have continued to flow despite accusations from Russian President Vladimir Putin that Russian gas sent to Ukraine by other purchasers violated agreement terms.
But volumes are being maintained by reverse flows and local output, according to Ukrainian Naftogaz CEO Andrei Kobolev.
According to reports on August 1, underground reserves stood at 14.9 billion cubic metres, against 14.3 billion at the start of July. This represented 46.6% of capacity.
Russian energy giant Gazprom halted delivery to Ukraine over lack of advance payment for June supplies specified in its contract with Naftogaz.
Ukraine owns 12 subsoil storage sites with an aggregate capacity of around 31 billion cubic metres, supplying Ukrainian needs and for gas transit onward to Europe.