Ukraine is pumping about 39 million cubic meters of gas into reserves every day, it said in a statement.
Ukraine has said it needs to collect about 19 bcm of gas in its storages, which are used to safeguard its own supply in winter as well as to ensure there are no disruptions to the transit of Russian gas across the country to Europe.
The European Union receives about 40 percent of its Russian gas imports via Ukraine.
Tensions between Russia, Europe's biggest gas supplier, and Ukraine, the main transit route to the EU, have been high since Moscow's seizure of Crimea in March 2014.
Moscow and Kyiv agreed last year on a "winter package" for supplies, with a discount of $100 per 1,000 cubic meters and advance payments.
That agreement expired on July 1 and last week, Russia, Ukraine and the European Commission initialled but failed to sign a new deal to secure winter gas supplies for Kyiv.
After five hours of talks in Brussels on Friday, European energy chief Maros Sefcovic said the technical details were agreed, but it would require "further procedures" before the deal could be signed.
Ukraine has said that $220 per 1,000 cubic meters was an acceptable price while Russia says Kyiv will pay around $230 in the fourth quarter, including a discount.