Apple has signed a long-awaited agreement with China Mobile to sell iPhones through the world's biggest network of mobile phone users.

In a deal that could add billions of dollars to its revenue, Apple said its smartphones will be available to China Mobile customers from January 17.

Pricing and availability details for the iPhone 5S and 5C lines will be disclosed at a later date, it said in a statement.

China Mobile, which has about 760 million customers, will begin registering orders for iPhone from December 25, the company said.

The tie-up provides a much-needed boost for Apple in a country where it is trailing rivals, even though China is its second-largest market after the US.

It will also give Apple extra firepower in its intensifying global competition with South Korea's Samsung Electronics.

Apple did not disclose financial terms of the agreement. Tim Cook, Apple's chief executive, said in its statement that China is an extremely important market for the California-based company.

In a country where smartphone sales are booming, Apple has trailed its competitors. Shipments of iPhones in the country grew 32% year on year for the third quarter, about half of China's Lenovo Group, which had the next slowest growth at 64% year-on-year.

China Mobile could gain 17 million new iPhone activations in 2014 alone, according to research firm Forrester. This would be more than double the 16.8 million iPhones Apple sold in mainland China for the 12 months ended September.

But after an expected initial surge, Apple is likely to find itself back in a costly marketing battle with Samsung Electronics. 

The deal has been years in the making, with numerous visits by Apple to the state-owned carrier's Beijing headquarters.

Negotiations have been tricky, in part because of disagreements over details like revenue-sharing, analysts have said.