U.S. stocks fell, with the Standard & Poor's 500 Index extending its steepest weekly drop since March, as Greece scrambled to avoid a cash crunch and energy shares tumbled with the price of oil.

Transocean Ltd. and ConocoPhillips fell more than 2.7 percent as West Texas Intermediate crude lost almost 8 percent. Aetna Inc. slid 6.8 percent, the most in three years, after agreeing to buy rival health insurer Humana Inc. for about $35 billion in cash and stock. Humana rose 1.5 percent.

While investors sold riskier assets after Greeks voted in a referendum Sunday to reject their creditors' austerity terms for aid, the declines were muted compared with a week ago. The onus is on Greece to act quickly to avoid a meltdown of its banks, which the government said will now remain shut through Wednesday. German Chancellor Angela Merkel and French President Francois Hollande are due to meet other euro-region leaders tomorrow.

Greek Finance Minister Yanis Varoufakis resigned, a move investors speculated may aid talks with creditors. Varoufakis said there was "a certain preference" among European creditors that he no longer be involved in negotiations.

U.S. stocks fell the most in three months last week as the escalating crisis in Greece stole attention from U.S. economic data and the Federal Reserve. The S&P 500 pared its 2015 gain to 0.9 percent, and finished Thursday 2.5 percent below its all-time high set in May.

The gauge suffered its biggest single-day decline of the year last Monday, down 2.1 percent, after Greece closed its banks and imposed capital controls.

The uncertainty in Greece overshadowed U.S. economic data that, while improving, wasn't strong enough to boost prospects for an increase in Fed interest rates. A report Thursday showed the U.S. economy added 233,000 jobs in June while wages stagnated and the size of the labor force receded.

Data Monday showed growth at U.S. service industries picked up in June from a more than one-year low, signaling steady improvement in the biggest part of the economy.

Investors will receive more signals on the health of the economy on Wednesday when earnings season kicks off and the Fed releases the minutes from its June meeting. The Federal Open Market Committee voted to keep the main rate at zero, where it has been since late 2008, as officials hold out for more decisive evidence of an economic rebound.

Quarterly earnings season begins that same day with Alcoa Inc. scheduled to report after the market closes. Analysts forecast corporate profits will contract 6.5 percent for the period before turning positive in the fourth quarter, according to data compiled by Bloomberg. Profits will end up growing 1.2 percent for the full 12 months, the data show.

All of the S&P 500's 10 main groups slid, with energy, raw-materials and industrial shares posting the biggest declines. WTI crude fell the most in seven months amid concerns about demand and ample supplies.

Transocean declined to a three-month low, and National Oilwell Varco Inc. lost 5.1 percent, the most in two months to pace energy losses after an industry report last week showed that the number of U.S. oil rigs in use advanced for the first time this year.

Metals miner Freeport-McMoRan Inc. retreated 3.7 percent to its lowest since March 20. Copper futures fell the most since January amid a slump in industrial metals as moves by China to halt a stock-market collapse and Greece's vote against austerity added to concern that global demand will ebb.

Industrial companies in the S&P 500 slumped to their lowest since January. Mining equipment maker Joy Global Inc. sank 3 percent to a nearly six-year low. Caterpillar Inc. and General Electric Co. lost more than 1.7 percent.