Taro Kono told Japanese parliamentarians on Tuesday there is a possibility North Korea has restarted operations at its nuclear facility in Yongbyon, local newspaper Nikkei reported.
Speaking before the parliamentary committee on diplomatic security, Kono said there are movements toward restarting the Yongbyon plant.
U.S. analysts wrote last week images from May indicate there were signs of activity at the North Korean nuclear research center.
"I will keep an eye on how much commitment North Korea makes toward denuclearization," Kono said, adding a campaign of pressure must be maintained until North Korea's commitment becomes clear.
The move comes ahead of Trump's expected summit with Kim Jong Un on June 12.
Following the summit, about a week later South Korean President Moon Jae-in could hold a meeting with Russian President Vladimir Putin, during the World Cup, Russian news service Izvestia reported Tuesday.
Topics to be discussed include three-way cooperation with North Korea.
While Tokyo remains firmly committed to sanctions against North Korea, the Chinese foreign ministry welcomed comments from Trump he may no longer be interested in using the term "maximum pressure" to describe U.S. policy.
Beijing's foreign ministry spokeswoman Hua Chunying said Tuesday China welcomes the move, according to South Korean news service News 1.
"We hope the two countries will communicate closely and make progress," Hua said.