"Right now we are in consultation with various members of the council on what steps might be taken because it obviously would be very serious," John Bolton told reporters at UN headquarters on Monday.
"But in any event we are just now in the preliminary consultations phase," he said.
Bolton's comments came after the United States, Japan and other countries warned North Korea against a missile launch as some officials said Pyongyang appeared to have finished fuelling for a test flight of a missile that could possibly reach as far as Alaska.
South Korean broadcaster YTN cited officials in Seoul as saying a launch of the North's Taepodong-2 missile was imminent.
Bolton said Washington did not really know at this point what North Korea's intentions were. "We need to wait for the event," he said.
"The first preference is that the North Koreans not light the missile off. We have made that clear to them," he said.
"Obviously we would like to know, if the North Koreans do light it off, what is under the nose cone."
In Washington, Bryan Whitman, the Pentagon spokesman, told reporters that US concerns about North Korean missile activities were "long-standing and well documented".
"The United States government has been consulting with allies in the region and has made clear to North Korea that a missile launch would be a provocative act," Whitman said.
The United States has found itself blocked by veto-wielding council members China and Russia in past attempts to raise North Korea's nuclear weapons programme in the Security Council.