US Secretary of State John Kerry Thursday dismissed a set of pre-conditions laid out by North Korea for talks as "unacceptable," calling them an opening "gambit" from Pyongyang.

The isolated North on Thursday responded for the first time to an offer from Kerry during his weekend visit to the Korean peninsula to return to the negotiating table in a bid to defuse heightened nuclear tensions.

The demands by the North's main military body included the withdrawal of UN sanctions and a permanent end to South Korea-US joint military drills.

"That's the first word of negotiation or thought of that we've heard from them since all of this has begun," Kerry told US lawmakers.

"So I'm prepared to look at that as... at least a beginning gambit -- not acceptable, obviously, and we have to go further."

The North's offer followed a month of increasingly hostile exchanges between Pyongyang, Seoul and Washington that have included threats of nuclear war and precision missile strikes.

During a trip to Seoul, Beijing and Tokyo, Kerry repeatedly stressed Washington was ready to talk to Pyongyang provided it was serious about reining in its nuclear program.

The US would not return to past cycles of "here's a little food aid, here's a little of this, then we'll talk," Kerry told the Senate foreign relations committee, adding "we've got to make some fundamental determinations here."

While in Beijing, Kerry sought to persuade China, North Korea's main ally, to use its leverage to defuse the tensions.

"One of the calculations I know that has been in Kim Jong-un's mind is that he can kind of do this and get away with it because he doesn't believe China will crack down on him," Kerry said.

"So that's a key consideration here and hopefully that in fact will be proven to be not true."

White House spokesman Josh Earnest also stressed to reporters aboard Air Force One that North Korea had to show it was serious about returning to talks.

"We're open to credible negotiations with the North Koreans, but we also need to see some clear evidence that the North Koreans themselves are willing to live up to their international obligations," he said.

Pyongyang must "demonstrate their commitment to ending the nuclear program, something they've promised in the past. And we haven't seen that thus far."