Russian gas monopoly OAO Gazprom (GAZP.RS) has cut gas supplies to Ukraine by 25% at 0700 GMT, the company said Monday.
"Gazprom is a reliable gas supplier, but it should not and will not supply gas without payment," said spokesman Sergei Kupriyanov.
The company said the cuts wouldn't affect supplies to customers in Europe.
Gazprom said Friday that Ukraine's supply would be reduced, saying the country has failed to act on a prior agreement between the Russian and Ukrainian presidents.