TOKYO, Dec 21 (Reuters) - Japanese government bonds were firm on Monday in line with slumping equities, while the longer zones extended gains made on Friday after the Bank of Japan pledged to increase the average remaining maturity of its JGB portfolio.

The benchmark 10-year JGB yield was flat at 0.265 percent, its lowest level since January.

March 10-year JGB futures were last nearly flat, down 0.02 point at 149.04, after they hit a record high of 149.18 on Friday.

The yield on the 20-year JGB fell half a basis point to 0.990 percent, its lowest since January, while the 30-year JGB yield slipped 1.5 basis points to 1.260 percent, its lowest since March.

On Friday, Japan's central bank kept intact its policy target of increasing base money at an annual pace of 80 trillion yen ($660.17 billion), and said it would extend the average remaining maturity of its JGB purchases to about 7-12 years from the beginning of next year, from its current target of about 7-10 years.

The Nikkei stock index was down 1.1 percent in afternoon trading.

Earlier on Monday, the BOJ offered to buy a total of 1.08 trillion in JGBs under its massive asset purchase programme, consisting of 350 billion yen of JGBs in the 1-year to 3-year zone, 350 billion yen of JGBs in the 3-year to 5-year zone, 240 billion yen of JGBs in the 10-year to 25-year zone, and 140 billion yen of JGBs maturing in over 25 years.

($1 = 121.1800 yen) (Reporting by Tokyo markets team; Editing by Sunil Nair)