The Pimco Total Return Fund, the world's largest bond fund, in June suffered its weakest monthly performance since September 2008 amid a selloff in U.S. Treasuries and other government bonds, data from Morningstar showed on Monday.

The fund, which has roughly $285.2 billion in assets, was down 2.64 percent last month, marking its weakest monthly performance since the 2007-09 financial crisis. The fund was ahead of just 17 percent of other U.S. intermediate-term bond mutual funds in June.

The Pimco Total Return ETF, meanwhile, was down 2.21 percent in June, marking its weakest monthly performance since inception in February 2012.

The DoubleLine Total Return Bond Fund, the flagship fund of the Los Angeles-based DoubleLine Capital LP, was down 1.74 percent last month — its weakest monthly performance since its inception in April 2010, Morningstar data showed.

That monthly performance bested 72 percent of peers, according to Morningstar.

The yield on the U.S. benchmark 10-year U.S. Treasury note rose about 36 basis points to 2.49 percent in June on fears that the Federal Reserve would scale back its $85 billion in monthly purchases of Treasuries and agency mortgage securities. Fed Chairman Ben Bernanke said on May 22 that the central bank could reduce its bond-buying later this year if the U.S. economy looked strong enough.

The Pimco Total Return Fund had 37 percent of its portfolio invested in Treasuries in May, its biggest position, according to Pimco's website. The DoubleLine Total Return Bond Fund had over 4 percent of its assets in 10-year Treasury notes as of June 4, DoubleLine Chief Executive and Chief Investment Officer Jeffrey Gundlach said on an investor webcast.