Steve Jobs was quoted in a press release and reportedly spotted at the Apple campus on Monday, raising speculation that the chief executive may have returned to work.
Apple Inc, which has not disclosed details on Jobs' state of health, declined to comment.
CNBC said Jobs, a pancreatic cancer survivor, reported to work on Monday, citing unidentified Apple employees who saw him. Jobs, 54, has visited the company periodically since starting his medical leave in January, according to blog reports that follow his every move.
This is the first time we've heard from Steve Jobs since he reported he was taking medical leave, said Oppenheimer & Co analyst Yair Reiner, referring to a press release from Apple on Monday that quoted Jobs. It's a sign Apple has its CEO back.
Apple said it had sold more than 1 million of its newest iPhone 3GS in the first three days of launch, beating analysts' expectations. Customers are voting, and the iPhone is winning, Jobs said in the statement.
Jobs has been on leave since January for an undisclosed condition, although the company has said he remains deeply involved in decision-making.
The Wall Street Journal reported on Saturday that Jobs had a liver transplant two months ago, but he was expected to be back at work before the end of June.
Shares of Apple rose as much as 1.5 percent in early trading before falling with the broader market to end the day down 1.51 percent at $137.37.
Analysts said the iPhone sales figures would distract some investors from concerns about Jobs' health to a certain extent.
Investors will realize that Apple executed well on the launch in Jobs' absence, said Susquehanna Financial Group analyst Jeffrey Fidacaro.
He said the 3GS sales were strong considering it was available in only eight countries. The last iPhone was launched in 21 countries and also booked 1 million sales in its first weekend. It shows that the iPhone momentum remains strong.
Despite the impressive weekend sales, Fidacaro said some investors would still have concerns about Jobs' health and whether he can continue as a driving force at Apple.
He's obviously the clear visionary of the company, Fidacaro said. There is a concern about his health and what Apple has told us about his health and what role he'll be playing when he returns.
Analysts have said that Jobs may soon shift to a new role focusing on big-picture issues and products at Apple, leaving Chief Operating Officer Tim Cook to manage day to day. Cook has been overseeing operations in Jobs' absence.
Shares of Apple have risen more than 60 percent this year as investors grow more comfortable with the executive bench beyond Jobs. Continued strong sales and product launches at the company have also helped.
On the one hand, you have the Jobs news out there, which might give people some pause, but on the other hand, iPhone 3GS sales were good, said Canaccord Adams analyst Peter Misek.
Six million customers have downloaded the new iPhone 3.0 software in the first five days since its release, Apple said.
The 3GS, which offers faster speeds, longer battery life and the ability to take videos, hit stores last Friday, drawing plenty of fans but not the crowds that had swarmed previous iPhone releases, due in part to preorders.
Exclusively on AT&T Inc's network in the United States, the iPhone competes with BlackBerry phones from Research in Motion and the Pre from Palm Inc
(Reporting by Clare Baldwin; Writing by Tiffany Wu; Editing by Steve Orlofsky)