Mitt Romney, the U.S. Republican Party presidential candidate, expressed concern Wednesday over North Korea's alleged sales of nuclear weapons.

He described the world as a "dangerous place" and stressed the seriousness of North Korea's threats.

"With instability in Pakistan and horrific violence in Syria, and with North Korea having shared nuclear technology, the threat of proliferation of weapons of mass destruction continues to be very real," he said, addressing a crowd in Indianapolis, Indiana, a day after he was formally nominated as the conservative party's flag bearer in the November presidential elections.

He did not elaborate which countries have nuclear ties with North Korea.

Romney said, "Major powers are rapidly adding to their military capabilities, sometimes with intentions very different from our own."

He also accused Iran of leading chants of "death to America" amid nuclear weapons ambitions.

With diplomacy sidelined in this year's U.S. presidential campaigns, the former Massachusetts governor has not laid out his clear vision yet on ways to deal with the Korean Peninsula.

In his official campaign Web site, Romney offered his basic view that tougher sanctions are needed against Pyongyang.

Romney would not risk causing a possible controversy by going deeper into specific foreign affairs before the elections, sources said.

"If he becomes a president, it's going to take some time to form his foreign policy team in earnest and present concrete diplomatic policies," a source close to the Romney camp said.

Rhetoric during campaign trails would not necessarily translate into actual policy, added the source.

Last week, Romney touched on the plight of North Korean people, highlighting the freedom and democracy of the U.S.

"Look around the world ... not a lot of dreamers in North Korea. The dreams they have there are just getting out," he said in Ohio.

With the Republican National Convention under way in Tampa, Florida, Romney is scheduled to deliver a key speech on Thursday to officially accept his nomination. (Yonhap News)