Japan has told an international meeting on North Korea that the world should not be blinded by Pyongyang’s recent “charm offensive”.
Foreign Minister Taro Kono’s comments in Canada come as North and South Korea discuss Pyongyang’s plans to participate in the Winter Olympics.
The 20 foreign ministers at the Vancouver talks agreed to increase pressure on Pyongyang.
But they also expressed support for the ongoing North-South discussions.
North Korea has in the past two years rapidly advanced its nuclear and conventional weapons programmes, despite increasing international sanctions.
Its latest ballistic test, on 28 November, sparked a new series of fresh sanctions from the UN, which targeted petrol shipments and travel.
But North Korean leader Kim Jong-un said in early January that he was “open to dialogue” with the South and proposed sending a delegation to the Games in Pyeongchang, South Korea next month.
The two Koreas subsequently held their first high-level talks in more than two years on how to make this happen.
The inter-Korean talks have been credited with easing tensions on the peninsula, but Mr Kono urged countries not to let their guard down.
“I am aware that some people argue that because North Korea is engaging in inter-Korean dialogue, we should reward them by lifting up sanctions or by providing some sort of assistance,” he said.
“Frankly I think this view is just too naive. I believe that North Korea wants to buy some time to continue their nuclear missile programme.”
The US echoed his remarks, with Secretary of State Rex Tillerson calling for an increase in pressure on Pyongyang to the point where it “must come to the table for credible negotiations”.
South Korean Foreign Minister Kang Kyung-wha told the meeting she believed recent sanctions and pressure had a part to play in North Korea’s recent willingness for talks.
“Indeed the concerted efforts of the international community has begun to bear fruit,” she said.
In their final joint statement, the 20 nations said they would consider imposing tougher sanctions on Pyongyang going beyond those required by UN Security Council resolutions.
The day-long meeting was co-hosted by the US and Canada. China and Russia, two of Pyongyang’s biggest allies, were not invited.
Both countries have been accused of not putting enough pressure on Pyongyang to stop it from developing its nuclear ambitions.
Beijing dismissed the meeting as “illegitimate”.
“The most important relevant parties of the Korean peninsula issue haven’t taken part in the meeting so I don’t think the meeting is legal or representative,” said Chinese foreign ministry spokesman Lu Kang.