Royce, chairman of the House Foreign Affairs Committee, led a bipartisan delegation to South Korea on Friday and Saturday.
His group met separately with President-elect Park Geun-hye and President Lee Myung-bak as they prepare for a power transition in late February.
"As Chairman of the House Foreign Affairs Committee, it is my intent to bolster this important relationship and take it to the next level," Royce was quoted as saying in the meetings.
Royce, known for his longtime support of the Washington-Seoul alliance, promised to work closely with the incoming Park administration not only on North Korea but also on major pending bilateral issues such as increasing the number of highly skilled visas for South Koreans in the U.S., his office said in an e-mailed press release.
"Chairman Royce recognized Korea's energy needs and agreed to take a fresh look at ongoing negotiations between the two countries regarding civil nuclear energy," it added.
The negotiations are one of the most urgent bilateral issues between South Korea and the U.S. as the deal, signed in 1974, is set to expire next year.
Seoul hopes to expand its non-military nuclear program but Washington is cautious, apparently due to the possible impact on its nonproliferation campaign.
On North Korea, which is apparently gearing up for another nuclear test, the allies will stand united, Royce stressed.
He also said he is looking forward to Park's trip to Washington later this year. No date has been fixed yet for her visit, though speculation is that it will happen in April or May.
"I will do my part to profile the importance of South Korea in the years ahead," Royce said.