A U.S. default is the biggest risk facing the global economy, a top Federal Reserve official said just hours after Fitch Ratings warned it could slash U.S. credit ratings if the government delays bond payments.

St. Louis Federal Reserve Bank President James Bullard told Reuters on Wednesday the U.S. fiscal situation, if not handled correctly, could turn into a global macro shock.

The idea that the U.S. could threaten to default is a dangerous one, he said in an interview.

The reverberations in those global markets would be very severe. That's where the real risk comes in, Bullard warned.

Some Republican lawmakers have said a brief default, which would be inevitable in August if lawmakers fail to raise the nation's $14.3 trillion debt ceiling, might be acceptable if it forces the White House to deal with large budget deficits.

Bullard's warning came just after Fitch said it would slash to junk the ratings on U.S. Treasury bonds, seen worldwide as a risk-free investment, if the government misses debt payments by August 15 in a row over slashing the budget's deficit.

The ratings would go back up once the government fulfills its debt obligations, but probably not to the current AAA level, Fitch said, in a stark statement about the impact of a short-lived default on the U.S. credit-worthiness.

The White House said Fitch's warning makes it clear that there is no alternative to raising the debt ceiling.

This is not about additional spending, this is about honoring the obligations the United States government has made, White House press secretary Jay Carney told a daily briefing.

Fitch's statement follows similar warnings by Moody's and Standard and Poor's, but Fitch was the first among the big-three rating agencies to say U.S. Treasury securities could be downgraded, even for a short period of time, to a non-investment grade.

Even a so-called 'technical default' would suggest a crisis of 'governance' from a sovereign credit and rating perspective, the agency said in a statement.

Fitch said it believes lawmakers will eventually agree on raising the debt ceiling, but added it was worried.

Clearly the political signals which are coming (from Washington) are a source of concern, David Riley, head of sovereign ratings at Fitch, told Reuters in an interview.

We know from previous experiences -- both with the government shutdown and previous episodes with the debt ceiling -- that although you get a lot of brinkmanship, ultimately it does get resolved, Riley said.

President Barack Obama is trying to win congressional approval to raise the nation's legal debt ceiling before an August 2 deadline.

The Treasury Department said on Wednesday the Fitch warning was another stark reminder of the need for Congress to act quickly.

PATH TO DEFAULT

Fitch said it would first place U.S. ratings on watch negative if lawmakers failed to enact an increase in the U.S. debt ceiling by August 2, the date when the Treasury Department will have run out of extraordinary measures to avoid a default.

The first test for U.S. ratings will come two days later when $30 billion in Treasury bills mature. If the government fails to repay them in full, Fitch will lower the rating on those specific securities to B-plus, four notches into junk territory.

But the real deadline comes on August 15 when $27 billion in Treasury notes and $25 billion in coupon payments come due. If the government misses those, Fitch would downgrade the U.S. sovereign issuer ratings to restricted default and lower all Treasuries securities to B-plus.

Though such an event (such as a short-lived Treasury bill default) may not permanently impair the capacity of the U.S. government to service its obligations, it is unlikely that its 'AAA' status would be retained in the short to medium term, it added.

Treasury Secretary Timothy Geithner has warned the United States could face a catastrophic default that would roil global markets if Congress does not raise the debt ceiling by then.

Moody's warned last Thursday that it could consider cutting the United States' top-notch credit rating if there was no progress by mid-July on a deal to reduce the deficit and raise the U.S. debt limit.

(Additional reporting by Daniel Bases and Burton Frierson; Editing by James Dalgleish)

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