* Fed easing will add to pressure on BOJ to ease next week * 20-yr sector underperforms ahead of next week's auction By Lisa Twaronite TOKYO, Dec 12 (Reuters) - Benchmark Japanese government bonds firmed slightly on Wednesday with yields creeping towards last week's 9-1/2-year low as investors await the outcome of the U.S. Federal Reserve's policy meeting for clues to how the Bank of Japan may act next week. Prices of 20-year bonds dipped, as investors prepared for next week's sale of that maturity. The Federal Reserve is expected to announce a new round of Treasury securities purchases at the conclusion of its two-day meeting on Wednesday. Half the respondents in a Reuters poll on Friday said they expected $45 billion per month. "I think people expect the Fed to increase their purchases in Treasuries, which is going to lead to balance sheet expansion, in which case the BOJ has to act, if they're going to play catch-up with the Fed," said Shogo Fujita, chief Japan bond strategist at Bank of America Merrill Lynch. "If they don't do it this month, it'll be next month, if they do it this month, they'll probably have to do it again in February," he said, referring to the BOJ, which meets Dec. 19-20. The BOJ will most likely increase its asset-buying and lending programme, currently at 91 trillion yen ($1.1 trillion), by another 5-10 trillion yen, sources have said. Japan's election on Sunday is also likely to set the stage for more pressure on the BOJ to take further easing steps. The opposition Liberal Democratic Party is likely to secure a majority in Sunday's general election and push the central bank to do more. "It is unlikely that there will be any big surprise from the election results, so the key event next week will be the BOJ meeting after it," said a fixed-income fund manager at a Japanese asset management firm. Yields on 10-year JGBs slipped half a basis point to 0.695 percent, moving closer to 0.685 percent touched last week, their lowest since June 2003. Yields on benchmark cash bonds ended 2011 at 0.980 percent. Ten-year JGB futures for March ended up 0.05 point at 144.70 point. Yields on 30-year JGBs were flat at 1.895 percent, while the 20-year yield added half a basis point to 1.660 percent. The Ministry of Finance will sell 1.2 trillion yen of 20-year debt on Dec. 18. The ministry will also offer 2.5 trillion yen of 5-year notes on Thursday. The 5-year yield was flat on Wednesday at 0.16 percent. On the data front, Japan's core machinery orders rose for the first time in three months in October, but fell short of forecasts. The JGB market showed no reaction to news that a rocket launched by North Korea on Wednesday appeared to have passed over Okinawa.