Collapsing energy prices have hit hard U.S. corporate junk bond funds, putting them on track for their worst performance in six years by squeezing the exploration and drilling companies that have been among most active issuers of high-yield debt.

U.S. funds have played a big part in bankrolling the U.S. shale boom, gobbling up billions of dollars of junk-rated debt issued by energy companies to rapidly expand production.

But in recent weeks, prices of some of these bonds have collapsed as oil prices have tumbled by a third since June to four-year lows, sapping energy companies' revenues and raising doubts about their ability to repay the debt.

Last week's decision by the Organization of the Petroleum Exporting Countries to refrain from output cuts to shore up prices only piled more pressure on the market.

"We have been concerned about the quality of the smaller E&P (exploration and production) players," said Ashish Shah, head of global credit at AllianceBernstein. "I think we're in the phase where people are selling what they can sell," Shah said, adding that his company had a light weighing on energy.

Many funds declined to comment on their exposure to the sector, but some have been clearly hit harder than the others.

Energy issues only make up 16 percent of nearly 2,300 issues covered by the Merrill Lynch High Yield Index.

But disclosures by major fund companies show many of the energy-related bonds they own belong to the most distressed category. It includes bonds with yield spreads of 1,000 or more basis points over benchmark U.S. government debt, considered a sign of financial strain. About one-third of the 180 total highly distressed bonds in the Merrill Lynch High Yield Index are energy issues with spreads greater than 1,000 basis points.

So far, high-yield mutual funds are up 2.82 percent this year, the worst performance since 2008, according to data from Lipper Inc, a unit of Thomson Reuters. Since the end of June the Merrill Lynch junk bond index has dropped 1.6 percent while its energy sub-index is down 7.1 percent.