Signs of strong demand for Apple's iPhone X led one well-known sector analyst to raise his forecast for pre-orders by 10 million on Monday and another to predict iPhone sales overall next year would be higher than consensus forecasts.

Apple has ceased to give its own numbers on iPhone sales, leaving investors and analysts striving to track output through research with its suppliers, consumer surveys and other industry indicators.

But the Cupertino-based firm did say on Friday that pre-orders for the 10th anniversary phone were "off the charts".

In a note on Monday, research house GBH Insights raised its pre-order demand expectations for the iPhone X to 50 million units from 40 million, calling the first stage of the iPhone X release a 'stellar success'.

Shares of the company gained another 0.7 percent in premarket trading after rising 3.6 percent on Friday.

"With the official launch of iPhone X in Apple retail stores slated for this Friday, November 3, we anticipate very high demand globally with limited supply of iPhone X on hand," Ives said.

Another analyst, Jeffrey Kvaal from brokerage Nomura Instinet, said Apple and U.S. wireless carriers' pushing out of delivery times for iPhone X orders to 5-6 weeks was longer than for previous phones and pointed to strong demand.

But a third, Drexel Hamilton analyst Brian White, cautioned that the longer wait times for consumers ordering phones may be as much due to the component supply issues which led Apple to delay the launch of the premium phone until November.

"Although we believe Apple is benefitting from strong demand from the iPhone X, the company is also struggling with supply constraints," White, who has a buy rating on Apple, said.

"A sound debate around the key driver for the surging shipping lead times can be made by reasonable people. Thus we believe it was important for Apple to highlight the demand side of the equation for the iPhone X."