Fitch Ratings has lifted its outlook for the U.S. banking industry to "stable," citing a near-doubling in first-quarter profits for the biggest banks compared with the same quarter a year ago.

The ratings agency had maintained a "negative" outlook for the industry since late 2007, as subprime mortgage troubles were spreading through the broader financial system.

Fitch said in its latest quarterly report on banks that earnings and revenues are still comparatively weak, and will remain under pressure over the next few quarters.

But quarterly income for the largest U.S. banks totaled $15.1 billion, compared with $7.6 billion over last year's first quarter, Fitch said. Fitch said it expects bank core earnings to show "slow but steady improvement" in coming quarters.

In assigning the stable outlook, Fitch cited recent stability in banks' non-performing assets, including loans that are seriously past due and in danger of default. Fitch also cited "vastly improved capital and liquidity positions" for larger U.S. banks.