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Apple unveils iPhone 6S, 6S Plus, aiming to tighten grip on high-end smartphones

The world's most valuable company will try to keep up the sales momentum of the iPhone -- its most important product -- with the new devices.

Apple's new iPhone 6S and 6S Plus will be available starting September 25. CNET

Apple on Wednesday debuted its newest smartphones, the iPhone 6S and iPhone 6S Plus, giving the devices a new pressure-sensitive display, zippier processor and more powerful camera.

"What we have to show you today is really awesome," CEO Tim Cook said during Apple's presentation at the Bill Graham Auditorium in downtown San Francisco. "While they may look familiar, we have changed everything about these new iPhones."

This year's iPhone is Apple's off "S" year. That means Apple keeps the basic design of the device the same but adds other features to attract buyers, like the Siri digital voice assistant in the iPhone 4S and the TouchID fingerprint reader in the iPhone 5S.

Now playing: Watch this: iPhone 6S and 6S Plus get 3D Touch, faster processors,...

The iPhone has become more vital than ever to the company's financial results following the introduction of Apple's bigger-screen smartphones a year ago. It has made up more than two-thirds of sales in the past several quarters since Apple released the iPhone 6 and iPhone 6 Plus in September 2014. Typically, the iPhone accounts for closer to 50 percent of Apple's revenue. That means keeping customers interested in the iPhone is critical for Apple to remain a major tech leader.

A marquee feature for this year's iPhone 6S was expected to be the Force Touch technology used in the Apple Watch, a pressure-sensitive display that responds to various types of touches. The company instead unveiled 3D Touch, a feature that appears to have the same capabilities. For instance, it will enable a user to peek in an email by pressing down on the subject head.

Using 3D Touch, users of the new iPhones can perform different actions based on the amount of pressure applied with a tap. James Martin/CNET

In another notable change, Siri can be activated via voice for the first time, using the command "Hey Siri." That will allow users to ask the digital assistant to play music or make a phone call, even while they are washing dishes or preparing a meal. In older models, Siri can only be activated by pressing the phone's home button. The new feature helps Apple catch up to other voice-activated devices such as the Amazon Echo speaker and many Android phones.

The new 4.7-inch 6S and 5.5-inch 6S Plus are the same size as last year's models. Preorders start Saturday and the devices go on sale September 25 in the US, UK, China, France, Japan, Australia and other markets. The phone will be available in 130 countries by year's end. Also, owners of newer models of the iPhone can upgrade to the new iOS 9 operating system starting September 16.

Following Apple's typical pricing system, the base-model iPhone 6S will cost $649 without a two-year carrier contract. Prices for the iPhone 6 and iPhone 6 Plus will be knocked down $100.

The new phones will include Apple's new A9 chip, which the company claims is 70 percent faster at computing tasks than the A8 chip in the iPhone 6 and 6 Plus. "It delivers a big jump in performance," Phil Schiller, Apple's senior vice president of worldwide marketing, said. "It's going to make using our phone faster and a lot more fun."

The devices also come with a new 12-megapixel iSight back-facing camera, a boost from the 8-megapixel cameras in last year's phones. The 6S and 6S Plus have a stronger glass cover and their bodies are made of a new aluminum alloy that's available in four finishes: silver, gold, space gray and a new rose gold. Additionally, the new phones will come with faster LTE and Wi-Fi wireless connections, a faster TouchID sensor and the ability to shoot high-resolution 4K videos. The phone's screen now acts as a flash for selfie pictures. Also, a new "Live Photos" feature will capture the moments just before and after a picture is taken, so a user can watch a short video version of any photo by pressing down on the picture.

The new phones come in silver, gold, space gray or rose gold. James Martin/CNET

Apple also unveiled a new program through its Apple Stores, which will allow people to upgrade to a new iPhone every year, with monthly installments starting at $32. That program should help keep the iPhone a more affordable option for consumers, as carriers continue moving away from two-year contracts and reveal the iPhone's expensive $649 base price tag in the process.

Because the new phone doesn't include major changes from last year's model, it's unlikely to reach the same level of record-breaking, runaway success as the iPhone 6 and iPhone 6 Plus. Those models' bigger screens helped attract tens of millions of new buyers, helping make the devices Apple's best-selling iPhones to date.

Yet even if the new 6S devices don't garner the same consumer attention, they should still help Apple maintain its stranglehold on high-end smartphones. The company gobbled up nearly all the operating profits available in the smartphone industry in the first quarter this year.

Apple's iPhone sales should remain strong in the coming year thanks to two major factors, said Carolina Milanesi, an analyst at Kantar Worldpanel. The install base for last year's blockbuster phones is still only 24 percent in the US and China, the device's two biggest markets. Additionally, those popular 2014 phones will soon become $100 cheaper, offering a big incentive to make a purchase, she said.

"There's a lot more people out there that are ready to upgrade," she said. "As much as the new products are important, the price of the older products is important."

However, worries have emerged that demand could be slowing down for Apple's biggest moneymaker. The smartphone market overall isn't growing as fast it once did. Shipments worldwide should rise only about 10 percent this year, according to IDC, well below the 28 percent increase in 2014, with China shouldering much of the responsibility for that slowdown. The smartphone market there has been maturing, and the country's stock market has been in turmoil amid concerns about an economic slowdown.

Cook, in a rare move, last month emailed CNBC host Jim Cramer to say that despite concerns in the Chinese stock market, Apple has "continued to experience strong growth for our business in China through July and August."

See all of today's Apple news.

Now playing: Watch this: The iPhone 6S and iPhone 6S Plus have arrived