UPDATE 1-S.Korea adopts toughest emissions cut goal-source

* South Korea adopts its toughest emissions target - source

* To cut 2020 emissions by 4 percent from 2005 levels

* Sets voluntary target as OECD's fastest-growing polluter (Adds quotes, details)

By Yoo Choonsik

SINGAPORE, Nov 14 (Reuters) - South Korea has adopted the toughest of its two voluntary 2020 emissions reduction targets, calling for a reduction of 4 percent from 2005 levels, a senior government source told Reuters on Saturday.

"The government has chosen the toughest option," said the source with direct knowledge of the decision. "I think it's important for countries to aim high."

The fastest-growing carbon polluter among the Organisation for Economic Cooperation and Development (OECD) plans to announce its decision next Tuesday, with the other option calling for keeping 2020 emissions unchanged at 2005 levels.

"There was some opposition from the industrial sector but the government's will is very firm on this issue," the official added, without elaborating on the reaction from companies.

Early this month, South Korea dropped the weakest of the three options calling for an 8 percent increase in 2020 emissions from 2005 levels.