Apple's iPhone 4 will continue to be in short supply in the domestic market until the end of this year, because of huge demand for the popular smartphone, China Unicom, Apple's sole partner carrier for iPhone on the Chinese mainland, said Monday.

China Unicom, the country's No. 2 mobile carrier, said it would freeze the deposit money of contracted iPhone 4 buyers if their phones are found to have been used without contracts.

Apple's iPhone 4 user base accounted for 10 percent of China Unicom's newly added 3G users, which reached 1.1 million last month, the carrier said.

"We are communicating with Apple to increase supply in the Chinese market but the shortage is expected to continue until the end of this year because of booming demand," China Unicom said in a statement.

At present, more than 600,000 people have pre-ordered contracted iPhone 4 models, with about 10,000 new pre-orders every day, according to China Unicom.

People can also buy iPhone 4 without contracts at Apple Stores - two each in Shanghai and Beijing - but they are often out of stock there.

From unauthorized channels, iPhone 4 often costs 20 percent more than the retail price that starts from 4,999 yuan (US$746).

Some China Unicom subscribers have sold their iPhone 4 for a neat profit and abandoned their contracts.

To prevent such behavior, China Unicom said it would check the iPhone 4 and contracts.

"If the iPhone 4 does not comply with our contracts, we will freeze the user's contract deposit money," it said.

By the end of October, China Unicom said its 3G user base was 11.65 million, a rise of 1.1 million in the month.