U.S. Treasury yields rose on Friday ahead of a keenly awaited speech by Federal Reserve Chairman Kevin Warsh, while 10-year German Bund yields increased to a 15-year high as eurozone bonds remained vulnerable to elevated energy prices.

All eyes were on the Federal Reserve Bank of Kansas City's Jackson Hole symposium, particularly Warsh's keynote speech at 1400 GMT.

Warsh has abandoned forward guidance, where the central bank provides an indication on the potential direction of interest rates. Still, investors will scrutinize his views on the economy and inflation. On the opening day of the Jackson Hole symposium on Thursday, some Fed members warned of the risks of inflation staying elevated, hinting at a possible need to raise interest rates in the coming months.

Boston Fed President Susan Collins said in an interview on the sidelines of the symposium: "I am open to supporting an increase [in interest rates] if I see conditions as not providing that evidence of continued disinflation that I'm looking for."

The 10-year Treasury yield rose 1 basis point to 4.682% during early European trade and the 30-year yield, which hit a 19-year high of 5.337% last week, was last up 1.4 basis points at 5.200%, according to Tradeweb.

The 10- and 30-year U.S. Treasury yields remained below recent peaks following the Treasury's decision last week to double the buyback of long-end securities to at least $4 billion from $2 billion per operation.

Impax Asset Management's fixed income CIO Ross Pamphilon said he doesn't expect policy signals from Warsh, who has been consistent in his skepticism of forward guidance.

Eurozone bond yields rose across the board, with the 10-year German Bund yield hitting a 15-year high of 3.276%, according to Tradeweb data. The 10-year gilt yield rose 2.1 basis points to 5.048%.

For eurozone bonds, negotiations on 2027 budgets will be a key risk and the talks could potentially refocus investor attention on fiscal challenges and political uncertainty, Societe Generale rates strategists said.

"Persistent geopolitical uncertainty and elevated oil and gas prices are not helping, alongside the global focus on growing sovereign debt, expectations of renewed supply, and increased political uncertainty after the summer recess," they wrote.

Long-dated Treasurys also remained vulnerable as investors continued to voice concerns that the increased buyback of long-end securities was not addressing underlying issues such as the U.S. budget deficit and mounting debt that now stands at $40 trillion.

"A buyback does not retire debt and so does not solve the structural problems that have been driving yields up," said Natalia Lojevsky, managing director at CIFC Asset Management, in a note.

Deficits, term premium and a heavy corporate supply calendar competing with Treasurys have pushed Treasury yields higher over the last two months, Impax Asset Management's Pamphilon said.