Ban Ki-moon became the first UN secretary general to visit the Pentagon Thursday, holding talks with US military leaders on the crisis over North Korea and a planned peacekeeping mission in Mali.

Ban had asked for the meeting amid mounting tensions on the Korean peninsula and drastic threats from the regime in Pyongyang, officials said.

Ban was welcomed with an honor guard at the steps to the Pentagon before a 30-minute meeting with Defense Secretary Chuck Hagel and the chairman of the Joint Chiefs of Staff, General Martin Dempsey.

The talks focused on North Korea as well as planned or potential United Nations missions in Mali, Somalia and Syria, a senior defense official, who spoke on condition of anonymity, told AFP.

"There was a lot of discussion on North Korea" and the risk of "misjudgment and miscalculation," the official said.

Ban, who served in the past as South Korea's foreign minister, has warned that a minor incident could trigger an "uncontrollable" situation after North Korea warned of impending nuclear war.

The Pentagon's intelligence reporting on North Korea grabbed headlines last week when a lawmaker revealed that the Defense Intelligence Agency had concluded Pyongyang likely had succeeded in producing a nuclear warhead that could be placed on a ballistic missile.

The assessment went further than previous statements by top officials about the state of the North's nuclear program but US intelligence officials later played down the report, saying it did not represent a consensus among the country's spy agencies.

Ban's visit came as North Korea responded for the first time to an American offer to return to the negotiating table, saying it would enter talks only if pre-conditions were met, including a withdrawal of UN sanctions and a permanent end to US-South Korea joint military exercises.

But South Korea promptly dismissed the response and US Secretary of State John Kerry called the North's stance "unacceptable."

Apart from North Korea, Ban's talks at the Pentagon covered a UN peacekeeping mission in Mali due to take over from French troops as well as "common US and UN interests" in Somalia and Syria, the US official said.

Ban's office said the secretary-general "expressed his condolences for the victims of the tragic bombing in Boston."