The selloff in global government bonds accelerated in European midday trade, pushing yields on 10-year German Bunds and U.S. Treasurys to fresh multiyear highs ahead of the European Central Bank's expected quarter-point rate hike and the U.S. Treasury's first buyback auction with increased volume.

Investors in eurozone bonds focus particularly on the ECB's monetary policy decision due at 1215 GMT and President Christine Lagarde's press conference after that. The near-complete certainty of a rate hike leaves Lagarde's remarks, particularly about inflation, as an important hint at the rate path ahead.

U.S. Treasury investors, meanwhile, will pay attention to producer price index data at 1230 GMT, a $22 billion auction of 30-year bonds, and to the Treasury's $6 billion buyback auction of 10- and 20-year debt--the first buyback operation with increased volume.

The 10-year German Bund yield rose to 3.451%, the highest since 2011, while the 10-year U.S. Treasury yield rose to 4.867%, an almost three-year high, according to Tradeweb. Joining the selloff, the 10-year U.K. gilt hit 5.295%, the highest since 2007.

For eurozone bond markets, "the ECB meeting is the obvious focal point today, but arguably the bigger question is whether anyone still has the conviction to fade the recent hawkish repricing," said Evelyne Gomez-Liechti, multi-asset strategist at Mizuho, in a note.

"The next question is not whether energy prices are high, but whether they generate meaningful second-round effects," she said. So far, the evidence remains limited, she added.

Thursday's widely anticipated interest-rate hike, which would bring the ECB's deposit rate to 2.50%, carries less importance than how willing Lagarde is to validate the market's recent pricing, Gomez-Liechti said.

"We expect upgraded inflation and growth forecasts, but not an outright endorsement of multiple additional hikes beyond September."

The elevated U.S. Treasury yield levels put focus on the first buyback auction with increased volume. The Treasury said in August that it would double the long-end nominal securities buyback volumes to at least $4 billion from $2 billion before. Wednesday's increase in U.S. Treasury yields after the Treasury's announcement of the $6 billion volume for Thursday's operation, however, might have signaled disappointment among investors who had looked for a higher volume.

"The price action could reflect some initial disappointment that the size of the bond buyback was not even bigger," Lee Hardman, senior currency analyst at MUFG, said in a note.

The increased buyback volume will be in effect for the remainder of the Treasury's current refunding quarter, through Nov. 4, it said earlier. It also said it would provide more information about future buyback sizes at the next quarterly refunding, scheduled for Nov. 4.

"Admittedly, it is highly uncertain how long the bigger purchases will be sustained and it is possible the size of operations could even be increased further going forward," Hardman said.

BondBloxx Investment Management's senior investment strategist, JoAnne Bianco, said investors might have been looking for a stronger signal that the Treasury was willing to meaningfully offset the supply pressures that have been pushing long-term yields higher.

"Instead, this announcement appears to be more of a liquidity management tool than a meaningful change in Treasury's broader debt management strategy," she said.

The scale of the Treasury's intervention would need to be substantial to ease pressure on long-end real rates, although the Treasury's position may help keep rate volatility subdued, Amundi said in a note.

"The Federal Reserve, meanwhile, is navigating a narrow path: inflation remains above target, but the labor market does not show signs of overheating," it said.