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The new iPhone 6S and iPhone 6S Plus are seeing robust initial demand, Apple said Monday.

Apple on Saturday began taking preorders for its latest iPhones, allowing consumers to lock in their purchases in advance of the actual launch date of September 25. The starting moments weren't entirely smooth: Some online sites, such as Apple's own online store, were unavailable for roughly the first hour.

But despite any technical issues, orders apparently have been flying.

Preorders for the two phones were "very strong around the world," and online demand for the 6S Plus was "exceptionally strong," said Apple spokesperson Trudy Muller. Based on the number of preorders, Muller added, Apple is "on pace" to surpass last year's initial sales when it moved 10 million units of the iPhone 6 and 6 Plus during their first weekend of availability (including more than 4 million preorders during the first 24 hours). That was double the initial weekend sales for the iPhone 5.

Muller didn't reveal the actual number of preorders received so far for the iPhone 6S and 6S Plus. The preorder estimate was first reported by the Wall Street Journal.

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Last year marked significant change for Apple as it launched its first big-screen smartphones in a challenge to sizable phones from Android rivals such as Samsung. The iPhone 6 and 6 Plus brought home a record 74.5 million iPhones during the last quarter of 2014, the first full quarter in which they were available. Apple even nudged past Samsung in smartphone market share that quarter, according to research firm Gartner, though Samsung took back the crown the following quarter.

This year marks an off-year for the iPhone, with less dramatic changes. One major new feature, called 3D Touch, lets you perform different actions on an app based on how hard you press its icon. But otherwise, the new iPhones mostly brought fine-tuning to the processor, camera, Touch ID, body and a few other features.

The launch numbers for the iPhone 6S and 6S Plus may get a boost from this year's inclusion of China, a huge market that wasn't in on the 2014 launch weekend, according to some Wall Street analysts.

That makes it trickier to do a precise year-to-year comparison, and the fact that Apple did not provide actual preorder numbers opens room for speculation.

"Typically Apple provides an update on the number of orders it receives during its first 24 hours," said Toni Sacconaghi, an analyst with the Wall Street firm Sanford C. Bernstein & Co. "However, Apple did not provide this figure today, raising the question of whether it *might* be lower than last year's total of 4 [million]."

The mobile landscape is also different from last year. In the past, people typically paid a subsidized price for their smartphones under a contract that restricted them to a new phone every two years. Now, all four of the major US carriers are either forcing or encouraging consumers to pay the full retail price for a phone, giving them the option to upgrade a new one more frequently. That means more buyers of last year's iPhone 6 or 6S may opt for the latest models if they're not tied to a contract. Apple itself now offers an annual upgrade in which buyers can pay off an iPhone with a certain amount each month in exchange for the option of buying next year's model.

At this point, Apple's US Web site is promising that most variations of the iPhone 6S will ship on September 25 to those who preorder, though some customers may have to wait two to three weeks. Demand for the iPhone 6S Plus, however, has created a delay in shipping, with most variations unable to ship for three to four weeks.

The iPhone 6S retails for $649 for the 16-gigabyte model, $749 for the 64GB version and $849 for the 128GB edition. The iPhone 6S Plus runs for $749 for the 16GB model, $849 for the 64GB version and $949 for the 128GB edition. The new phones will be available in 12 countries at Apple retail stores starting at 8:00 a.m. local time on September 25.