WASHINGTON (Yonhap) -- The United States openly expressed "extreme concern" Monday over North Korea's possible proliferation of weapons.

"We remain extremely concerned about North Korea's proliferation activities and related equipment," State Department spokesperson Victoria Nuland said at a press briefing.

She was responding to a news report on the communist nation's alleged arms sales to Syria and Iran.

Nuland neither confirmed nor denied the report involving the two nations.

"North Korea has for many years tried to market its missile technology and its equipment," she said. "And these exports undermine security in the region, and they earn revenue that the regime uses to fuel its own weapons program. So it's very dangerous activity."

Earlier in the day, Republican Sen. John McCain demanded that Washington toughen sanctions against North Korea, already subject to strong United Nations sanctions for its nuclear and long-range missile tests.

He also urged Beijing to use its political and economic influence on Pyongyang.

"The only country that can really force change in North Korea is obviously China," he said in a speech on U.S. policy toward Asia at the Center for Strategic and International Studies, a think tank in Washington.