BEIJING, Dec 18 (Reuters) - China's state planner on Monday said it had asked national energy majors CNPC, Sinopec and CNOOC to cut natural gas supplies to some industries by a total of about 15 million cubic metres per day.

The move comes amid a shortage of gas in the world's No.2 economy after an unprecedented campaign to help clean up the environment by switching millions of households to natural gas from coal for heating.

The government has previously said that supply to industry would be curbed, but Monday's announcement - covering makers of products such as chemicals, methanol and fertiliser - gives an indication of the targetted size of such cuts.

Meng Wei, spokeswoman for the National Development and Reform Commission (NDRC), also said at a briefing on Monday that the government had coordinated with the three major energy companies to increase supplies by raising gas output, boosting imports and speeding up infrastructure construction.

She added that China planned to purchase 3.5 billion cubic metres of liquefied natural gas in overseas markets in addition to its original plan to import 24.5 billion cubic metres over the winter.

The nation has also been sending 14 million cubic metres of gas per day to its frigid north from the south, with plans to boost that to around 19 million tonnes per day, said Meng.

CNOOC has rented a convoy of 100 trucks to ship liquefied natural gas (LNG) the length of the country to northern regions in the latest unusual, and costly, move to ease the deepening fuel crisis.