BANGKOK, Jan 11 (Reuters) - Thailand will extend its ban on smoking to air-conditioned bars and offices and outdoor markets next month, joining major developed countries in a war on tobacco, a leading Thai anti-smoking agency said on Friday.

Smokers faced a 2,000 baht ($60) fine and owners who fail to enforce the law could be fined 20,000 baht ($600) after the ban takes effect on Feb. 17, said the Thailand Health Promotion Institute, which helped push for the law.

The Health Ministry ban, already in place for air-conditioned restaurants, could face opposition from some bar owners, group president Hatai Chitanondh said in a statement.

"They may be thinking the Health Ministry is ruining their business, kicking away their customers, but our research papers show a smoking ban will bring more customers to them," Hatai said.

The move comes hard on the heels of similar bans this month by France and Germany. Many other members of the European Union have also outlawed smoking in enclosed public places.

Ireland imposed the world's first nationwide ban on smoking in the workplace in 2004. U.S. states, including Florida and California, have had similar bans since 2003.

In November 2004, Bhutan became the first country to ban tobacco sales entirely.

(Reporting by Nopporn Wong-Anan; Editing by Michael Battye)