Under the deal, which was due to be signed today, Russian energy giant Gazprom will acquire a 50 percent stake in Belarus's gas pipeline monopoly, Beltranshaz.
Belarus's government spokesman Alyaksandr Tsimashenka said negotiations are continuing.
Gazprom spokesman Sergei Kupriyanov downplayed the last-minute delay, saying the deal would be signed by June 1.
RFE/RL Belarus, Ukraine, And Moldova Report
SUBSCRIBE For weekly news and analysis on Belarus, Ukraine, and Moldova by e-mail, subscribe to "RFE/RL Belarus, Ukraine, And Moldova Report."