As the U.S. Food and Drug Administration considers whether to approve the innovative Iqos smoking device, made by Philip Morris International Inc., researchers have raised concerns about studies submitted to the authority, half of which were conducted in Japan.

With a decision expected this year, former employees and contractors have described a number of irregularities involving clinical trials that underpin the tobacco giant's application to the agency.

By heating tobacco instead of burning it, the company says its Iqos device avoids subjecting smokers to the same levels of carcinogens and other toxic substances found in a regular cigarette. The company has spent more than $3 billion developing new smoking platforms like Iqos.