Credit Suisse cut its year-end target on the S&P 500 index to 1,450 from 1,470, saying the probability of a complete break-up of the euro zone has risen by 10% from 5%.
The chances of Greece leaving Europe's single currency bloc is now up at 15% from 5%, Credit Suisse said, adding that a Greek exit would be "extremely damaging" for the country.
The European Central Bank (ECB) will not renegotiate Greece's bailout package and there are no alternatives to sticking with it if Greece wants to stay in the euro zone, ECB executive board member Joerg Asmussen said on Tuesday.
But Greek political parties remain deeply divided over sticking to a programme that saved Greece from bankruptcy but caused steep economic hardship. Most believe cuts demanded by the EU and International Monetary Fund are only making the situation worse by increasing unemployment.
The euro, world shares, precious metals and oil all fell on Wednesday as Greece's political deadlock raised investor concern that the country will destabilise the euro zone as it did in 2009 when it triggered the bloc's debt crisis.
Credit Suisse, however, said it is still "overweight" on equities for the next three to six months on the back of impressive growth in the United States and easing fiscal policy in China.
Equities are to some extent less risky than governments equivalents, and earnings revisions are still positive, the brokerage noted.
The S&P 500 Index fell 0.43% to 1,363.72 on Tuesday.
