Speaking on condition of anonymity, the official told AFP that if the test goes ahead, it would "probably" occur on July 27, which is the 64th anniversary of the signing of the Korean Armistice Agreement.
The date is a public holiday in the nuclear-armed North and celebrated as Victory Day.
The official said the test would be of either an intermediate-range missile or North Korea's ICBM -- known as a KN-20 or a Hwasong-14.
Experts assessed it could have put Alaska in range, bringing Pyongyang's long-held dream of a missile that can deliver an atomic warhead to the United States within reach, and presenting President Donald Trump with a stark challenge.
More From This Section
South Korea's news agency Yonhap quoted a government source as saying Seoul had seen North Korea moving transporter erector launchers carrying ICBM launch tubes in North Pyongan province.
"They're setting up for something," a second US defense official told AFP.
After North Korea's July 4 test, the United States launched a push at the United Nations for tougher measures against Pyongyang.
The United States has been locked in negotiations with China for nearly three weeks on a new raft of measures, and Haley said China was negotiating with Russia separately on possible tougher sanctions.
"I think we are making progress," Haley said.
"It's not as fast as I would like but these are pretty serious sanctions and so I think that there is a lot of thought going into this."
Haley told the UN Security Council after the test that she hoped to present new measures in a few days, such as cutting off oil supplies, banning North Korean guest workers or imposing new air and maritime restrictions on North Korea.
Haley said the true test will be Russia's role. Moscow maintains the July 4 launch was not an ICBM, citing its defense ministry's assessment.
Moscow has also argued that sanctions are not the answer to rein in North Korea, and that talks, as advocated by Beijing, were needed.
North Korean leader Kim Jong-Un, who personally oversaw the July 4 launch, described it as a gift to the "American bastards."
Kusong has been the scene of past tests, including in May when an intermediate-range ballistic missile traveled more than 700 kilometers (435 miles).
The North last week refused to respond to the South's offer to open dialogue to ease tension.