HANOI, Sept 6 - Vietnam's Nam Con Son gas pipeline will be shut from Friday for periodic maintenance, cutting natural gas supplies to power plants responsible for 13.5 percent of the country's electricity output, a company official said on Tuesday.

"The pipeline will undergo the maintenance, which is conducted every five years, so it will be closed totally between Sept. 9 to 14," said an official of the Nam Con Son Pipeline Co, which is 51 percent owned by Petrovietnam Gas.

Maintenance at gas suppliers' exploration areas in the Nam Con Son basin started on Monday for completion on Sept. 19, the official said.

As a result, gas supply has been reduced to around 11.5 million cubic metres (cbm) per day, from 21.5 million cbm previously, state utility Vietnam Electricity group said in a separate statement.

Electricity transmission had been increased from the northern region to the southern region, while generation had been maximised from other thermal power plants.

Gas-fired power plants using Nam Con Son gas account for 40 percent of the electricity capacity in the southern region, home to Vietnam's many major industrial zones.

Vietnam produced an estimated 7.26 billion cbm of natural gas in January-August, up 4.2 percent from a year ago, while electricity output in the same period rose 11.6 percent to 116.2 billion kilowatt hours, government data showed.

Maintenance at the Nam Com Son gas supply chain was last carried out in late 2011.