NEW YORK, Dec 30 (Reuters) - The U.S. Treasury yield curve flattened on Wednesday afternoon as traders bought longer-dated debt to rebalance their portfolios ahead of the month-end. The 30-year yield was last down 1.4 basis points at 1.660%, pulling the spread between five- and 30-year yields flatter to 129.5 basis points. That spread had hit a high of 131.2 basis points earlier in the day. The benchmark 10-year yield was a basis point lower at 0.925%, bringing the two-year, 10-year spread to 80.3 basis points. Traders must rebalance their portfolios at the end of the month, which can sometimes lead to market moves that are out of sync with news and economic data. The maturity of a bond portfolio falls at the end of the month as bonds move closer to their maturity date. In order to rebalance, traders buy longer-dated debt, which drives yields lower. “The sense on the desk is that much of what they saw today may have been month-end related flow,” wrote Bill O’Donnell, rates analyst at Citigroup, noting that traders may have been moving a day early with some international markets closed on Thursday. The flattening in afternoon trade reversed a slight steepening trend earlier Wednesday. Though investors bought U.S. Treasury debt in the Asian and European sessions, investors sold Treasuries in the North American session. It was the third consecutive day in which that trend had played out, said Tom Graff, head of fixed income at Brown Advisory. “I think U.S. traders are starting to bet on a very boring 2021,” said Graff. “U.S. traders figure if the Fed is on the sidelines, nothing is changing.” Meanwhile, he said, non-U.S. traders may be concerned about the downsides of the Brexit deal signed on Wednesday, U.S. coronavirus relief or other issues. The chances of bigger stimulus checks for Americans dimmed on Tuesday after Senate Majority Leader Mitch McConnell put off a vote on increasing the COVID-19 relief checks from $600 to $2,000. Volume on Wednesday was thin and likely to remain so in the United States and Europe this week, with some European markets closed on Thursday and all markets shut on Friday for New Year’s Day. “The market is as quiet and low volume as it has been in a very long time,” said Justin Lederer, Treasury analyst and trader at Cantor Fitzgerald. “The only headline out there that would be a large market mover would be if the $2,000 stimulus checks were passed.” (Reporting by Kate Duguid; editing by Jonathan Oatis)