"Japan, Korea and the USA continue to agree that pressure is needed to be applied on North Korea", Onodera told reporters after his meeting with Mattis and South Korean Defense Minister Song Young-moo on the sidelines of the Shangri-la Dialogue.
"We will continue to implement all U.N. Security Council resolutions on North Korea", Mattis said as he wrapped up a visit to Singapore Sunday.
U.S. Defence Secretary Jim Mattis said on Sunday that North Korea will receive relief only after it shows "verifiable and irreversible" steps toward denuclearization, adding that it would be a bumpy road to a summit between President Donald Trump and North Korean leader Kim Jong Un on June 12, Reuters reported.
Trump announced the cancellation of the summit, scheduled to take place in Singapore, at the end of May in a letter to Kim.
"If we believe North Korea will trick us in the future as it did in the past, how can we negotiate with it and make peace?" the minister said at a plenary session of the forum.
What's most important for now is to provide every support needed for the success of the North Korea-U.S. summit talks, he told reporters later.
The U.S. has previously insisted that North Korea give up all its weapons before it can shed its pariah status or get any relief from sanctions.
"In light of how North Korea has behaved in the past, I believe it is important not to reward North Korea exclusively for agreeing to have a dialogue".
He pointed out, however, that the North has taken some "positive measures" recently, apparently referring to the release of three American prisoners and the purported demolition of its nuclear test site in Punggye-ri.
Mattis made similar comments when he met with defense ministers in Singapore.
Amid growing speculation that North Korean leader Kim Jong Un will call for a reduction or withdrawal of USA military forces in South Korea at the summit with Trump, the three defense ministers also exchanged views on the US military footprint in East Asia.
Concerns have risen among the US' regional allies in recent days that Trump might consider cutting a deal with Kim that would see Pyongyang agreeing to give up its long-range missiles in return for sanctions relief, while being allowed to hold on to its short-range arsenal, leaving Japan and South Korea in the cross hairs.
Trump and Kim have been supposed to meet in a summit in Singapore on June 12.
Onodera and Song were to hold bilateral talks separately later Sunday.
Song was quoted by the official as telling Onodera that given the rapid pace of developments on the Korean Peninsula, South Korea wants to cooperate with Japan to achieve the complete denuclearization of North Korea.