ATHENS (Reuters) - There has been no clear progress on tackling the euro zone crisis in the last two weeks, but Europe's leaders understand the high cost of failing to act, Vice President Joe Biden said on Sunday as he arrived in Athens in a show of support for struggling Greece.

"I think the clearer it becomes that there's no alternative and time is running out, not just in Europe but in the United States and the world ... is when you're more likely to get some very difficult decisions made," Biden told reporters travelling with him from Istanbul to Athens.

"But beyond that, I have no empirical data to suggest that we should, or should not be, more encouraged today than two weeks ago," he said.

The euro zone faces a crunch week as leaders gather for a summit and the U.S. Treasury Secretary flies in to consult, in a graphic demonstration of Washington's concern that the crisis could undermine a fragile U.S. recovery if it is not tackled decisively.

(Reporting by Alister Bull)