DoubleLine Capital's co-founder Jeffrey Gundlach warned on Tuesday that it might be premature for the U.S. Federal Reserve to raise interest rates next month, given junk-bond prices are hovering near four-year lows.

"To raise interest rates when junk bonds are nearly at a four-year low is a bad idea," Gundlach said in a telephone interview.

Gundlach, widely followed for his prescient investment calls, said if the Fed begins raising interest rates in September, "it opens the lid on Pandora's Box of a tightening cycle."

Gundlach said the selling pressure in copper and commodity prices driven by worries over China's growth outlook "should be a huge concern. It is the second-biggest economy in the world."

Last year, Gundlach correctly predicted that U.S. Treasury yields would fall, not rise as many others had forecast, because inflationary pressures were non-existent and technical factors, including aging demographics, were at play.

The Los Angeles-based DoubleLine Capital had $76 billion in assets under management as of June 30.

The DoubleLine Total Return Bond Fund, DoubleLine's largest portfolio by assets and run by Gundlach, had positive inflows in July.

The Total Return fund attracted a net inflow of $390.4 million last month, compared with $81.7 million in June. It has $47.2 billion in assets under management and invests primarily in mortgage-backed securities.