The United States has military options for dealing with North Korea that wouldn't put South Korea’s capital of Seoul at risk, Defense Secretary James Mattis said Monday.

“Yes there are. But I will not go into details,” Mattis told reporters at the Pentagon.

Mattis wouldn’t say any more about the military plans, but did confirm that he discussed with his South Korean counterpart the idea of introducing nuclear weapons to the Korean peninsula. He would not confirm if that was one of the options under consideration.

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He also said diplomacy and sanctions are working in pressuring Pyongyang.

The United States and its allies have been scrambling to respond to North Korea’s accelerating nuclear program, which appears to be growing more sophisticated.

North Korea on Friday launched its second ballistic missile over Japanese airspace in a month.

Mattis said the Pentagon has not attempted to shoot down the two recent missiles, nor any others, because they have not posed a direct threat, The Associated Press reported.

He added, however, that North Korea is deliberately avoiding crossing a line that would “make them vulnerable,” and that U.S. and Japanese missile defenses have been ready to respond.

It “would elicit a different response from us,” if a North Korean missile were to threaten U.S. or Japanese territory, he said.

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U.S. Ambassador to the United Nations Nikki Haley said Sunday that if diplomatic measures with North Korea continue to fail, Mattis is ready to “take care” of the situation.

President Trump has repeatedly refused to rule out a military strike on North Korea's nuclear facilities.

His national security adviser, H.R. McMaster, reiterated Sunday on ABC's "This Week," that "all options are on the table" when it comes to addressing the North Korean threat.