Greece's prime minister says the cash-strapped country could earn up to 150 billion euros ($206 billion) in three decades from offshore oil and gas deposits.

Antonis Samaras said Wednesday the rough estimate, which is worth nearly half of Greece's 320 billion-euro debt mountain, was "neither optimistic nor pessimistic," and could increase depending on further exploration results.

He said surveys have provided "very strong indications of significant" reserves off Greece's western coast, and work is ongoing in waters off the island of Crete in the south.

Substantial findings would be a major windfall for a country that has been in recession for over five years.

Greece has been forced to cut spending sharply, raise taxes and enact wide-ranging economic reforms to secure vital rescue loans after it nearly went bankrupt.