US stocks tumbled Friday on fresh worries about a Greek debt default as US President Barack Obama called on Greece to enact reforms.

At the closing bell, the Dow Jones Industrial Average was at 17,826.30, down 279.47 points (1.54 percent).

The broad-based S&P 500 fell 23.89 (1.13 percent) to 2,081.10, while the tech-rich Nasdaq Composite Index dropped 75.98 (1.52 percent) to 4,931.81

Obama said that debt-wracked Greece needs to take action to bring stability to its shaky finances. Uncertainty about Greece has hung over the International Monetary Fund and World Bank spring meetings this week.

"Greece needs to initiate reforms," Obama said at a joint press conference with Italian Prime Minister Matteo Renzi at the White House.

"They have to collect taxes. They have to reduce their bureaucracy, (institute) more flexible labor practices."

European stocks also fell sharply amid Greece concerns, with Germany's DAX 30 index tumbling 2.58 percent and France's CAC 40 in Paris slumping 1.55 percent.