The five U.S. regional banks most exposed to the energy sector set aside millions of dollars more in the first quarter to cover potential losses from bad loans to the struggling oil industry.

Overall, provisions for bad loans at Comerica Inc, Regions Financial Corp, KeyCorp, SunTrust Banks Inc and Fifth Third Bancorp more than doubled to $570 million in the period from $222 million a year earlier.

"The low oil price environment continues to pressure our energy clients, which contributed to the increase in non-performing loans, our reserves and provision expense," William Rogers, chief executive of Atlanta-based SunTrust said on a post-earnings call on Friday.

"We remain focused on being proactive around energy, and therefore have increased the resources and intensity around mitigating our risk and helping our clients navigate through this downturn," Rogers added.

About 2.2 percent of SunTrust's total loans were linked to the energy sector as of Dec. 31, according to Barclays.

Dallas-based Comerica said this week its energy loans accounted for about 6 percent of its total loans. The bank was identified by Barclays as having the biggest exposure to energy of any U.S. bank as a percentage of total loans at year-end.

"Should prices remain in the $30 to $45 range, we continue to expect losses in the $50 million to $75 million range in 2016," David Turner, the chief financial officer at Regions, said on a call with analysts last week.

Birmingham, Alabama-based Regions had energy-linked exposure of 3.9 percent of total loans at the end of the 2015, according to Barclays.

KeyCorp, headquartered in Cleveland, and Fifth Third, which is run out of Cincinnati, had exposure of 2 percent and 1.8 percent respectively.

U.S. banks are already under pressure from near-zero interest rates, forcing them to resort to cost-cutting to underpin earnings as revenue growth remains weak.

Investors and regulators worry that depressed oil prices are not only putting companies at risk but are leading to job losses that threaten regional economies, adding to the banks' problems.

If oil slipped back to the $30-per-barrel level hit in January, from about $45 now, there could be more pain to come for the banks. Oil traded at more than $100 per barrel in 2014.

Up to a third of all oil producers may end up in bankruptcy this year if weak oil prices continue to crimp their access to cash and ability to cut debt, according to a study by consulting firm Deloitte published in February.

Oklahoma City-based oilfield services company Seventy Seven Energy Inc is the latest company to say it planned to file for bankruptcy protection. Dozens of others already have.

"I don't see the problem resolving itself in the second half of 2016 unless we see a sudden uptick in the price of oil," said John Orton, chief financial officer at Amplify Credit Union in Austin, Texas.

Total non-performing assets of the five regional banks rose 60 percent to $4.35 billion in the first quarter.

Three of the five reported a drop in earnings. SunTrust and Regions Financial reported a rise in profit, helped by higher interest income.

Comerica reported the biggest drop in quarterly profit among the three. The Dallas-based lender's net profit fell 55 percent as it boosted bad loan provisions by more than 10 times to $148 million.

Reporting by Sweta Singh