Oil futures settled at their lowest level in three weeks on Monday as uncertainty surrounding the financial crisis in Greece helped send prices lower for a fourth straight session.

On the New York Mercantile Exchange, West Texas Intermediate crude for August delivery fell $1.30, or 2.2%, to settle at $58.33 a barrel. Prices logged their lowest settlement since June 8. August Brent crude fell $1.25, or 2%, to $62.01 a barrel on London’s ICE Futures exchange.

The Greek government has called a referendum for Sunday and has shut down the banking system as it tries to resolve its five-year debt crisis. The move puts Greece closer than ever to an exit from the euro, raising doubts about the future of the common currency. While there is no direct impact from Greece on oil markets, analysts said the uncertainty is having a knock-on effect in most financial assets. “The ongoing uncertainty caused by Greece kick-started a flight from crude at … and news of progress relating to Iranian sanctions has only served to endorse this move,” said Matt Smith, director of commodity research at Clipper Data. “Markets hate uncertainty, and the possibility of Greece exiting the euro has markets rattled.” Analysts have also said that the Greek financial turmoil has raised worries about the prospects for energy demand.

U.S. stocks were sharply lower in the wake of the unraveling crisis in Greece, while the euro edged higher against the U.S. dollar, but the U.S. dollar index traded mostly lower. Treasury yields tumbled. “What happens in the currency, equity and bond markets can be very important for oil. Could oil go its own way?” asked David Hufton of PVM in London in a research note.

Meanwhile, the oil market has again largely ignored last week’s terrorist attacks in Kuwait and Tunisia. The market is keeping an eye on Iran, where Tuesday’s deadline for the Persian nation to reach an agreement over its nuclear program is likely to be extended. If an agreement can be reached, it could allow Iran to begin exporting crude to global markets, which would pressure oil prices further.