By Alina Selyukh

WASHINGTON, Dec 12 (Reuters) - Major U.S. wireless carriers on Thursday pledged to make it easier for consumers to "unlock" their mobile phones for use on competitors' networks, responding to pressure from consumer groups and the top U.S. communications regulator.

Verizon Wireless, AT&T Inc, Sprint Corp, T-Mobile US and U.S. Cellular agreed to "clearly notify" customers when their devices are eligible for unlocking and to process unlocking requests within two business days, said wireless industry group CTIA.

U.S. wireless carriers often "lock" smartphones to their networks as a way to encourage consumers to renew their mobile contracts. Consumers often get new devices at a heavily subsidized price in return for committing to longer contracts.

The top carriers have long allowed consumers to unlock devices and take them to another network at the end of a contract term - commonly, two years - though the process varies by company and can be quite painstaking.

Then in late 2012, the Library of Congress, the minder of U.S. copyright law, completed a new triannual review of exemptions to the law that effectively made phone unlocking illegal, even after the consumer completed the contract.

The ruling surprised many telecom observers, outraged phone users, and finally landed on the White House's agenda thanks to an online citizen petition that gathered 114,322 signatures, more than the 100,000 needed to spur a response. The White House sided with the petitioners.

Unlocking then became a top 2013 policy matter for new FCC Chairman Tom Wheeler, a former CTIA chief for whom it presented an opportunity to distance himself from his former industry.

In November, he sent a letter to the CTIA demanding the carriers voluntarily agree to unlock phones for customers in good standing to ensure phone users still have that option.

On Thursday, the agreement was announced and the five carriers pledged to unlock devices after the customer's contract is fulfilled, including pre-paid ones within a year of purchase. (To read the agreement, see: bit.ly/1h3XS1G)

"Today was an important day for consumer choice," Wheeler said on Thursday. "Today's commitment by wireless providers will provide consumers with more information about when and how to move their devices from one compatible network to another, should they decide to do so."

The agreement was welcomed by Sina Khanifar, one of the organizers of the White House petition, although he and public interest groups would still like a permanent change to the copyright law and more flexibility for consumers to unlock their phones before their contracts end - possibly without having to ask the carriers' permission first.