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TREASURIES-U.S. yield curve flattens ahead of Trump speech

* U.S. Treasuries produce biggest monthly gain since June * Investors await details on fiscal stimulus from Trump speech * Traders raise view on Fed increasing U.S. rates in March * U.S. Q4 GDP holds at 1.9 pct, falling short of forecast (Update market action, add quote) By Richard Leong and Sam Forgione NEW YORK, Feb 28 (Reuters) - Longer-dated U.S. Treasury yields fell on Tuesday on month-end buying and shorter-dated yields rose on bets the Federal Reserve migh raise interest rates as soon as March, causing a part of the yield curve to hit its flattest level since November.

The Treasuries sector was on track in February to post its biggest monthly total return since June, according to an index compiled by Bloomberg and Barclays.

Investors awaited possible details on tax cuts, reduced regulation and infrastructure spending from U.S. President Donald Trump who was scheduled to speak before a joint session of Congress at at 9 p.m. (0200 GMT Wednesday).

Treasury yields have retreated since mid-December when benchmark yields reached their highest in over two years on expectations of swift implementation of fiscal measures pledged by Trump during his campaign last year.

Details on these programs, which investors had bet on to cause a surge in federal debt and domestic inflation, have been scarce. Some of them hoped Trump's upcoming speech will offer specifics on his perceived pro-growth economic agenda.

"The bond market is waiting and looking at the uncertainty that things may take longer-than-expected," said Andrew Richman, director of fixed income at SunTrust Advisory Services in Jupiter, Florida.

The benchmark 10-year Treasury yield was marginally lower in late trading at 2.365 percent, while the 30-year yield was down nearly 2 basis points at 2.967 percent.

In contrast two-year yield, which is most sensitive to traders' view on Fed policy, was up over 3 basis points at 1.236 percent.

The spread between two-year and 10-year yields shrank to 112 basis points, its tightest level since November.

A number of Fed officials have said the U.S. central bank is considering raising interest rates soon as the economy approaches full employment and its 2 percent inflation goal.

San Francisco Fed President John Williams said a rate hike at the Fed's March 14-15 meeting is up for "serious consideration," while Philadelphia Fed chief Patrick Harker said he was clinging to his outlook for three rate increases in 2017.

Interest rates futures implied traders saw about a 37 percent chance on a March rate hike. The over-the-counter swap market suggested traders assigned a 60 percent probability for a such a move.

Some analysts remained skeptical of a rate increase in two weeks as economic data depicted only a modest pace of expansion that doesn't require a faster pace of rate hikes.

Earlier Tuesday, the Commerce Department left its fourth-quarter reading on the gross domestic product at 1.9 percent, falling short of analyst forecast. February 28 Tuesday 4:03PM New York / 2103 GMT Price US T BONDS MAR7 152-31/32 0-11/32 10YR TNotes MAR7 125 -0-36/25 6 Price Current Net Yield % Change (bps) Three-month bills 0.555 0.5635 0.035 Six-month bills 0.7125 0.725 0.033 Two-year note 99-200/256 1.2362 0.032 Three-year note 99-168/256 1.494 0.035 Five-year note 99-224/256 1.9013 0.033 Seven-year note 99-138/256 2.1964 0.013 10-year note 98-252/256 2.365 -0.002 30-year bond 100-160/256 2.9683 -0.017 DOLLAR SWAP SPREADS Last (bps) Net Change (bps) U.S. 2-year dollar swap 35.00 -0.75 spread U.S. 3-year dollar swap 27.75 -1.25 spread U.S. 5-year dollar swap 11.50 -0.75 spread U.S. 10-year dollar swap -2.50 0.00 spread U.S. 30-year dollar swap -38.00 0.25 spread (Additional reporting by Sam Forgione; Editing by Frances Kerry and Chizu Nomiyama)