"As many customers noticed, the online demand for iPhone 6S Plus has been exceptionally strong and exceeded our own forecasts for the preorder period," the company also wrote. "We are working to catch up as quickly as we can, and we will have iPhone 6S Plus as well as iPhone 6S units available at Apple retail stores when they open next Friday."
The company's release this year differs somewhat from 2014, when it announced how many units it sold in the first 24 hours of preorders.
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Tech analyst Walter Piecyk at BTIG said Monday it's not clear how to compare initial preorders to 2014.
"Last year, it started on a Thursday night at midnight. This year, because of a late Labor Day, is was Friday night," he told CNBC's "Squawk Box" in an interview. "So we don't know what the comp is, whether they have to wait through the end of Monday to find out exactly what the actual number is going to be."
From a feature standpoint, Piecyk said the new Force Touch incorporated in the iPhone might not be enough on its own for users to upgrade: "But from a functionality [standpoint], it's actually great."
The force-sensitive technology 3D Touch allows what Apple calls "peek and pop," a way for users to preview content with a light touch or open it with a heavier touch.
"It is really going to change the way people use the iPhone," Piecyk said. "It's going you save you time, and that's what people really care about."
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Meanwhile, FBR Capital Markets senior analyst Daniel Ives said the preorders suggested a strong trajectory for the iPhone 6S and iPhone 6S Plus.
He said he was encouraged by the response in China, where Apple's website showed particularly long wait times for the phones. "It shows that they are off to a white-hot start, with China really being front and center as a main driver of initial demand."
The company's stock has sunk in recent weeks on concerns about how it will fare amid the downturn in the Chinese economy. "Hopefully this can start to turn the negative tide that's been overhanging the stock," Ives said.
During the market turmoil on China concerns and Apple's growth prospects there, CEO Tim Cook sent an email to CNBC's Jim Cramer on Aug. 24, saying, "We have continued to experience strong growth for our business in China through July and August."
"As you know, we don't give midquarter updates and we rarely comment on moves in Apple stock," Cook also wrote at the time. "But I know ... [the] question is on the minds of many investors."
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Early indications the new iPhones are also selling well in China bodes well, Piecyk said Monday.
"[But] Cook put his reputation on the line by sending Cramer the email saying that China's fine. If the numbers don't deliver through that's his reputation that's on the line," he contended.