GENEVA (Reuters) - The United States is optimistic about an eventual return to six-party talks on North Korea's nuclear program but more time is needed to build a basis for resumption, the chief U.S. negotiator said on Tuesday after two days of talks with North Korean negotiators.

"It has been a very useful meeting," U.S. negotiator Stephen Bosworth told reporters outside the U.S. mission in Geneva.

"The tone was positive and generally constructive," he added. "I am confident that with continued effort on both sides we can reach a reasonable basis of departure for formal negotiations for a return to the six-party process."

(Reporting by Stephanie Nebehay, editing by Tom Miles)