TOKYO, June 25 (Reuters) - Japanese government bond yields ticked higher for a fourth straight session on Tuesday, as investors positioned for a looming reduction in the Bank of Japan's bond buying.

Investors were also cautious amid uncertainty over future bond supply, following recent reports that the finance ministry may shorten the average maturity of JGB issuance.

A smooth auction of 20-year JGBs on Tuesday helped to cap the rise in yields.

The 20-year JGB yield added 1.5 basis points (bps) to 1.84% as of 0605 GMT, while 30-year yield also rose 1.5 bps to 2.19%.

The 10-year yield was 1 bp higher at 0.995%.

Benchmark 10-year JGB futures finished the day down 0.1 yen at 143.48.

The BOJ is due to reveal its plans for a tapering of its massive monthly bond buying programme, known locally as rinban, at the conclusion of its next policy meeting on July 31. The central bank has also kept the door open to an interest-rate hike at the same gathering.

"Long-end yield are poised to gradually rise until we get a clearer picture of what's going to happen with supply and rinban," said Shoki Omori, chief Japan desk strategist at Mizuho Securities.

"There are not many investors involved in the longer end, so volatility there is going to rise."

The five-year JGB yield rose 0.5 bp to 0.545%, while the two-year yield fell 0.5 bp to 0.295%. (Reporting by Kevin Buckland; Editing by Rashmi Aich)