* Fading supply concerns underpin superlong tenor * 10-, 20-yr yield spread narrows from last week's record TOKYO, Jan 29 (Reuters) - Benchmark Japanese government bonds slumped, taking their cue from U.S. Treasuries which dropped in the previous session, as investors awaited this week's U.S. Federal Reserve meeting for clues on how long easy policy will continue. But the superlong tenor fared better on expectations of duration extension buying, as well as fading supply concerns after local media reports that next fiscal year's new JGB issuance will be less than some market participants had feared. The 10-year JGB yield rose 2 basis points to 0.765 percent. Last week, the benchmark yield fell to a six-week low of 0.720 percent. "Profit-taking is continuing in 10-years, after it strengthened too much at the end of last week, and particularly after the 10-year Treasury yield went above 2 percent yesterday, with investors waiting for the FOMC," said a fixed-income fund manager at a Japanese asset management company. "But the Bank of Japan's easy policy continues to support JGBs, particularly short- and medium-term maturities," he added. The five-year yield was flat at 0.155 percent. On Wednesday, at the conclusion of its two-day meeting, the Fed's policy statement might contain clues on whether its latest bond purchase program could end as early as this year. By contrast, the Bank of Japan said last week it was doubling its inflation target to 2 percent and making an open-ended commitment to buying assets beginning next year. The benchmark 10-year U.S. yield broke above 2 percent on Monday for the first time since last April. The 10-year JGB futures contract ended down 0.23 point at 144.07. On Friday, futures rose as high as 144.58, their highest level since Dec. 13. Superlong maturities outperformed on expectations of some duration extension buying in the final trading days of January. Funds often buy to extend the duration of their portfolios. Market participants also cited expectations that new bond issuance in fiscal 2013 will be less than some had initially feared. Later on Tuesday, the government will release its plan for JGB issuance in the fiscal year that will begin from April. New bond issuance will likely be less than 43 trillion, according to recent local media reports. "The additional increase in the superlong sector in the new fiscal year JGB issuance plan will prove limited," said Naomi Muguruma, a senior fixed-income strategist at Mitsubishi UFJ Morgan Stanley Securities. The 20-year yield added half a basis point to 1.775 percent, while the 30-year bond yield slipped half a basis point to 1.990 percent. The spread between the 10- and 20-year yields, which widened to a record high of 104.5 basis points last week, narrowed to 101.0 basis points on Tuesday.