WASHINGTON - Serious delinquencies among U.S. prime mortgages rose nearly 20 percent in the third quarter from the prior quarter, as the percentage of current and performing mortgages fell for the sixth consecutive quarter, banking regulators said on Monday.

The report by the Office of Comptroller of the Currency and the Office of Thrift Supervision, which are part of the Treasury Department, covered about two-thirds of all U.S. mortgages.

It found 3.6 percent of prime mortgages -- those made to the most credit-worthy borrowers -- were seriously delinquent in the third quarter. That was more than double the year-ago quarter and up nearly 20 percent from the 2009 second quarter.

The report defined serious delinquencies as those loans 60 days or more past due and loans to delinquent bankrupt borrowers.

Big U.S. banks and thrifts carried out 2.4 million home loan modifications, trial period plans or payment plans in the quarter, spurred mostly by a government plan offered by President Barack Obama, according to the report.

Most came from the government's Home Affordable Modification Program. Mortgage servicers carried out 274,000 trial plans in the third quarter, up 240 percent from the second quarter when the plan was launched.

But only 1 percent of those had been converted to permanent modifications as of September 30, 2009, the report said.

A major cause of this disconnect is that loan servicers are finding that many borrowers who initially appear to qualify for the program do not, according to the report.

The Treasury Department has been pressuring lenders and mortgage servicers to do more to ease the harm from rising foreclosures.

Loan modifications made outside the new aid program fell in the third quarter by nearly 8 percent, the report said.

(Reporting by Kim Dixon, editing by Matthew Lewis)