Talks resumed last week in Beijing among U.S. and North Korean officials. They were the first since North Korean leader Kim Jong Il died in December.
Glyn Davies, the U.S. special envoy for North Korea, described talks as "substantive" but said it was premature to make any assessments on resuming multilateral talks aimed to convincing the North Koreans to abandon their nuclear ambitions.
Davies said in Tokyo that he was "able to go into some depth about our mutual positions and try to narrow differences a little bit," but added that "only time will tell" how well the negotiations went.
A spokeswoman for the U.S. State Department said "no decisions" were made on whether six-party talks would resume or whether food assistance was offered to the North Koreans.
South Korea's Yonhap news agency reports the North Koreans are interested in resuming six-party talks that broke off more than three years ago.
U.S. and South Korean militaries started military drills in the region Monday involving around 225,000 troops. The North Korean government, as it generally does when such exercises begin, said it viewed the games as preparation for a military invasion.
North Korea is "fully ready for both dialogue and war," the South Korean news agency quoted Pyongyang as saying.