Several missile experts have since reached a similar conclusion, but they caution the significance of last week's successful ICBM launch -- North Korea's first in more than four years -- should not be discounted, pointing out the test still demonstrated a weapon with the theoretical ability to hit all of the continental United States.
The ICBM fired by North Korea last Thursday flew to an altitude of 6,000 kilometers (3,728 miles) and to a distance of 1,080 kilometers (671 miles) with a flight time of 71 minutes before splashing down in waters off Japan's western coast last Thursday, according to Japan's Defense Ministry.
Japan's Vice Defense Minister Makoto Oniki told reporters shortly afterward that the missile's altitude would suggest it is a "new type of ICBM."
Japanese officials were sticking to that assessment this week, with Chief Cabinet Secretary Hirokazu Matsuno saying on Monday that Tokyo judged the missile to be a new type based on flight altitude and other information.
And CNN previously reported that the Pentagon is still assessing to what extent the missile is an improved version of previous launches.
But the South Korean official and missile experts said further close analysis of images in North Korean state media of last week's launch gave two potential clues relating to Pyongyang's alleged subterfuge.
The South Korean official said assessments by Seoul and Washington showed the ICBM launched last week only had two engine nozzles, like Hwasong-15, whereas Hwasong-17 has four.
And video released last Friday by state-run Korean Central Television (KCTV) purporting to show Kim Jong Un guiding the launch reveal the North Korean leader's shadow appearing westward, meaning it was filmed in the morning, but the launch took place in the afternoon, the official said.
Also, it was cloudy in the launch area last Thursday, but the weather in the KCTV video appears to be sunny, the official said.
Analysts say US must still be wary
Several missile experts have also begun to cast doubt on North Korea's claim to have launched a Hwasong-17.
"North Korea released a video after the March 24 test. We measured the shadows in it, however, and it is clear from the altitude and angle of the sun that the video is from the test on the morning of March 16," Lewis said.
"The video is of the (previous) test that failed. That strongly suggests the other test was something different that they don't want us to see."
Ankit Panda, a nuclear policy expert at the Carnegie Endowment for International Peace, said Pyongyang may have altered the Hwasong-15 -- first tested nearly five years ago -- to make it look like a more powerful missile.
Panda said Pyongyang's apparently inflated claim was aimed at a domestic audience rather than internationally.
On Tuesday, South Korean lawmaker Ha Tae-keung told reporters that, according to a military briefing, debris rained down over the North Korean capital, Pyongyang, following the failed March 16 test.
Ha's office confirmed the lawmaker's remarks to CNN on Thursday, adding North Korea may have been prompted to announce the March 24 Hwasong-15 launch as a Hwasong-17 to temper negative opinions in Pyongyang, where citizens witnessed the March 16 failure. North Korea has not acknowledged reports of a failed March 16 test.
Lewis, the nuclear weapons expert, said regardless of which missile was fired last Thursday, the test showed a powerful offensive capability that US defense officials have to be wary of.
"The missile fired on March 24 would have had a range of about 12,000 kilometers (7,500 miles), which is certainly within the capability of a Hwasong-15, which can deliver a nuclear weapon anywhere in the United States," Lewis said.
And Matsuno, the Japanese official, said Monday that North Korea's missile program remains a serious threat to the security of Japan, the region and the world.
CNN's Yoonjung Seo and Junko Ogura contributed to this report.