FOREX-Euro pressured by Spain downgrade

* Euro resumes declines, hits one-year low

* S&P downgrades Spain's credit rating

* IMF sees aid to Greece worth 100 bln-120 bln euros

(Adds detail, updates prices)

NEW YORK, April 28 (Reuters) - The euro touched a one-year low against the U.S. dollar after Standard & Poor's downgraded Spain's credit rating, snapping earlier bursts of optimism that had put the single currency in see-saw trade.

S&P cut its ratings on Spain by one notch to AA from AA-plus, citing a more protracted period of sluggish economic growth than previously expected. The outlook for the country is also negative. For details, see [ID:nWNA9804]

News of an imminent aid package to debt-stricken Greece helped the euro earlier. The credit downgrade renewed risk aversion on investor worries the euro zone's debt crisis is spreading. The downgrade on Spain followed one day after S&P had slashed its ratings on Portugal.

Analysts said the downgrades on Portugal and Spain reflect wider credit risks in the euro zone, which could create cracks in the euro system and further batter the single currency.

"There's no upside potential for the euro right now, seriously" said Win Thin, a currency strategist at Brown Brothers Harriman & Co. in New York.

"Any aid package now will come in too late," he said. "If the help had arrived a couple of months earlier, it would have prevented the situation from deteriorating further not only in Greece but in other countries such as Portugal and Spain."

In early afternoon New York trade, the euro was little changed at $1.3158 EUR=, after hitting a one-year low at $1.3114, on electronic trading platform EBS. The low was $1.3116 according to Reuters data.

Earlier, the euro rose as high as $1.3268 on news a euro zone/International Monetary Fund aid package for Greece was imminent and may be worth more than previously expected.

The aid package for Greece will be worth 100 billion euros to 120 billion euros ($132.6 billion-$158.8 billion) over three years, according to IMF Managing Director Dominique Strauss-Kahn, a member of German parliament for the opposition Greens said. For details, see [ID:nBAT005353]

Strauss-Kahn later said details of the plan would not be available until talks in Athens are concluded. [ID:nBEB004456]

Both Strauss-Khan and German Chancellor Angela Merkel said on Wednesday negotiations with Greece will be concluded in coming days. [ID:nBEB004458]

"The tone (was) a tad more upbeat" earlier, said Amelia Bourdeau, a currency strategist at UBS AG in Stamford, Connecticut, but added that any rebound in the single currency would be short-lived.

The euro has fallen below long-term support levels in recent months, including $1.3405, a 61.8 percent Fibonacci retracement of its rally from its 2008 trough to its 2009 peak. This points to the possibility of further declines. A 76.4 percent retracement lies around $1.2990.

In the United States, the Federal Reserve is widely expected to keep U.S. interest rates on hold near zero after a two-day policy meeting that ends later on Wednesday, and stick to its commitment to keep them there for an "extended period". [ID:nN27160583]

Elsewhere, the dollar rose to its highest against the Swiss franc since July 30, 2009, according to Reuters data. It climbed as high as 1.0924 before retreating to 1.0887, little changed on the day.

(Reporting by Nick Olivari and Vivianne Rodrigues; Editing by Andrew Hay)