* Greece faces default on 1.6 bln euros
* Euro STOXX 50 index set for worst day since 2011
* S&P futures volume twice the average at this time of day
* Futures down: Dow 186 pts, S&P 22.50 pts, Nasdaq 56.25 pts (Adds details, comment, shares)
By Sweta Singh
June 29 (Reuters) - U.S. stock index futures fell around 1 percent in heavy trading on Monday after Greek bailout talks broke down, intensifying fears that it could be the first country to exit the euro zone.
Markets around the world fell on fears the problem could spread to other European countries. The blue-chip Euro STOXX 50 index towards its biggest one-day percentage loss since late 2011.
The European Central Bank froze funding to Greek banks, forcing Athens to shut banks for a week to keep them from collapsing.
"What the Greek government is doing is almost unprecedented for developed nations," said Adam Sarhan, chief executive of Sarhan Capital in New York.
"I don't remember anytime in recent history where a developed nation literally shut down its banking system for an entire week."
Greece faces default if it does not repay 1.6 billion euros ($1.8 billion) to the International Monetary Fund on Tuesday.
Default would send Greece sliding towards a euro exit and challenge Europe's grand project to bind its nations into an unbreakable union with a common currency.
Adding to the uncertainty, Chinese stocks closed sharply lower after a volatile day of trading despite surprise monetary easing by the central bank.
U.S. investors also await May data for pending home sales from the National Association of Realtors, which is expected to have dropped 1.2 percent after rising 3.4 percent in April. The data is expected to be released at 10 a.m. ET (1400 GMT).
The Federal Reserve Bank of Dallas will issue manufacturing outlook survey for June at 10:30 ET (1430 GMT).
Investors have been keeping a keen eye on data to see if the U.S. economy has recovered from a slow start at the beginning of the year. The Federal Reserve has said it will raise rates when it sees a sustained rebound in the economy.
A September interest rate hike is "very much in play" if the U.S. economy continues to strengthen, though the Federal Reserve could also wait until December to start tightening policy, New York Fed President William Dudley told the Financial Times in an interview.
S&P 500 e-minis were down 22.5 points, or 1.07 percent, with 399,460 contracts traded - twice the average volume at this time of the day.
Nasdaq 100 e-minis were down 56.25 points, or 1.26 percent, on volume of 47,304 contracts. Dow e-minis were down 186 points, or 1.04 percent, with 46,328 contracts changing hands.
Sysco shares rose 2 percent to $38.99 in premarket trading after the largest U.S. food distributor said it terminated its merger with privately held U.S. Foods.
Seres Therapeutics fell 10.5 percent to $46 after an 86 percent rise in its market debut on Friday.
Aratana Therapeutics rose 15 percent to $15.50 after the animal health drugmaker said its experimental appetite-stimulating drug was found effective in dogs in a pivotal study.
Insurer Assured Guaranty fell 19.7 percent to $22.00 on Monday after BTIG downgraded the stock on concerns over Peurto Rico's debts. (Additional reporting by Siddharth Cavale in Bengaluru; Editing by Saumyadeb Chakrabarty)