Iran’s ambassador to Moscow said Monday that Russia could build a second nuclear reactor at Iran’s Bushehr nuclear power plant in exchange for Iranian oil.

Russia and Iran have been negotiating a deal that could see 500,000 barrels of Iranian oil per day exchanged for goods and equipment, reports Reuters. Mehdi Sanaei, the Iranian ambassador, told a Russian news agency that proceeds from the sale of oil could pay for Russian contractors to build a second reactor at the power plant.

The news comes a day before Iran is set to resume talks in Vienna with six leading world powers, including Russia, to agree on a a permanent deal to curtail Iran’s nuclear program in exchange for lifting international economic sanctions.

Western nations have expressed fears that any Russia-Iran deal to exchange oil for goods could jeopardize chances for securing a nuclear agreement. Iran’s Supreme Leader Ayatollah Ali Khamenei said on Monday he was not “optimistic” about the negotiations set to begin Tuesday.

[Reuters]