(Bloomberg) -- Japan’s 20-year bond yields rose to the highest level since 2015 as global debt markets come under increasing pressure due to expectations of further monetary-policy tightening.

Yields on the securities extended gains even after the Bank of Japan announced unscheduled bond-purchase operation, climbing four basis points to 1.03%. The benchmark 10-year yield rose 0.5 basis point to 0.25%, the upper limit of the trading range the BOJ tolerates under its yield-curve-control policy.

So-called super-long bonds are more susceptible to moves in overseas debt as they are outside the BOJ’s area of curve control. While Tuesday’s 40-year debt sale drew robust demand, concerns persist that the central bank’s outsize presence in the debt market is preventing arbitrage trades between cash bonds and futures, worsening market dislocation.

“The effect of the BOJ’s unscheduled debt buying will just evaporate if bond yields overseas continue to rise,” said Ataru Okumura, a strategist at SMBC Nikko Securities Inc. in Tokyo. “It’s becoming difficult to use futures for hedging before and after a bond auction, resulting in a decrease in volumes and liquidity as well as a rise in volatility.”

The difference between cash bonds and futures has expanded in recent weeks and is now close to the widest in three months. In June, the BOJ’s buying of cheapest-to-deliver securities sparked a blowout in arbitrage trades involving bonds and futures, leading to a surge in the so-called basis.

In Tuesday’s unscheduled debt-purchase operations, the central bank bought 150 billion yen ($1.04 billion) of notes due in five to 10 years, and 100 billion yen of 10-to-25-year bonds. The BOJ also conducted an unlimited debt-buying operation targeting futures-linked securities and 10-year notes, but not for super-long debt.

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