NEW YORK (AFP) - Tick, 20,000 dollars, tock, another 20,000 dollars.

So rapid is the rise of the US national debt, that the last four digits of a giant digital signboard counting the moving total near New York's Times Square move in seemingly random increments as they struggle to keep pace.

The national debt clock, as it is known, is a big clock. A spot-check last week showed a readout of 8.3 trillion -- or more precisely 8,310,200,545,702 -- dollars ... and counting.

But it's not big enough.