A planned merger by Switzerland's Holcim Ltd and France's Lafarge to create the world's biggest cement maker might hurt competition in India, the country's antitrust regulator said on Saturday, asking the firms for more details.

The all-share combination between the two companies, which requires approval from regulators around the world, already has the green light from seven countries.

The Competition Commission of India (CCI) "formed a prima facie opinion that the combination is likely to have an appreciable adverse effect on competition," the government said in a statement.

The regulator asked the two companies to publish details of the deal on their websites as well as publish them in four leading daily newspapers and sought comments or objections from the public within 15 days of merger details being published.