NEW YORK (TheStreet) -- U.S. stocks extended their losses Monday as Greece's debt woes pushed crude prices to their lowest level in almost three months. 

The S&P 500 fell 0.79%, the Dow Jones Industrial Average lost 0.66%, after hitting its lowest point on Monday since Feb. 3, and the Nasdaq declined 0.82%.

The drama in Greece fanned fears of lower global economic growth, which sent oil prices sharply lower by 7.75% to $52.52 a barrel and marking the commodity's biggest drop in nearly three months. The looming Tuesday deadline for nuclear negotiations with Iran also put downward pressure on prices. This sent shares of energy stocks lower. Transocean (RIG) fell 4.8%.

About 61% of voters in Greece rejected a bailout package demanded by Greece's European creditors and the International Monetary Fund. The "no" vote on Sunday leaves the possibility of a "Grexit" wide open, as experts said Greek banks have just days of liquidity remaining. A "Grexit" would mean a return to the drachma for Greece or the creation of an alternative currency.

Greece Finance Minister Yanis Varoufakis resigned on Monday in an attempt to give his government more leverage in negotiations, which have been heated under his tenure. Reports said Euclid Tsakalotos, a member of the Greek parliament since 2012 and the nation's bailout negotiator, will succeed Varoufakis. 

Meanwhile, German Chancellor Angela Merkel, whose nation holds the largest amount of Greek debt, flew to Paris on Monday to meet with French President Francois Hollande to discuss how debt talks will proceed, if at all. She said the door is still open for debt talks with Greece and proposals "must be on the table this week," according to CNBC.

Stocks in Europe were lower. London's FTSE 100 dipped 1.4%, Frankfurt's DAX lost 1.5% and Paris's CAC fell 2%. The euro lost 0.24% against the dollar.

"An exit of Greece from the eurozone might actually strengthen the euro and enable the eurozone to focus on getting the economy back up and running," Alastair McCaig, a market analyst with London-based IG Markets, told TheStreet TV. "I think there's going to be plenty of buying opportunities in the not too distant future. Europe's quantitative easing program hasn't really been felt because of Greece, but once it is, there will be attractive equity opportunities for U.S. investors in the European arena."

Kraft Heinz began trading on the Nasdaq on Monday. In 2013, Warren Buffet of Berkshire Hathaway took over H.J. Heinz and merged it with Kraft Foods. Shares of Kraft Heinz gained 1.7%.

Health insurance giant Aetna (AET) agreed to purchase Humana (HUM) in a deal announced Friday for roughly $230 a share, or $27 billion. Shares of Aetna fell 7.4%, while Humana gained 1.7%.

Discount retailer Dollar Tree (DLTR) has been given the go-ahead from the Federal Trade Commission for its $9.2 billion takeover of Family Dollar (FDO) after selling 330 locations to Sycamore Partners, a private-equity firm. Shares of Dollar Tree lost 0.7%.

Shares of Weight Watchers International (WTW) soared 16% after The New York Post reported an activist investor is looking to purchase a majority stake in the struggling company, which has seen its shares slump 84% year to date.