The number of companies defaulting on their debt has risen this year to the highest since the financial crisis, according to S&P Global Ratings.

The ratings company said four more businesses were downgraded to default this week, three of them from the oil and gas industry, sending the total to 57 issuers. That’s the highest for the period since 2009, it said.

U.S. businesses and firms in the energy, metals and natural resources sectors accounted for the majority of defaults, according to S&P Global Ratings. Commodities companies have struggled to overcome a collapse in oil prices as producers increased output amid slow economic growth worldwide.

“We expect credit to remain under pressure in this sector, leading to even more defaults,” said Diane Vazza, head of S&P global fixed-income research.