The Marlboro cigarette manufacturer Philip Morris International has acknowledged "serious concerns" after activists found children as young as 10 picking tobacco destined for the global company in dismal conditions in a remote region of Kazakhstan.

The US-based Human Rights Watch has documented 72 cases of child labour, ranging in ages from 10 to 17, on Kazakh farms in the district of Enbekshikazakh, where Philip Morris is the sole purchaser of tobacco.

Hellish Work‚ a study published today , examines the conditions of migrant workers, estimated to number between 300,000 and 1 million, many of whom travel without visas from neighbouring countries such as Kyrgyzstan to seek work in Kazakhstan's tobacco industry. Some have their passports confiscated by employers and are only paid at the end of each season in a lump sum, making it hard to escape the harsh conditions. There were cases of "debt bondage" whereby workers ended a season owing their employers money, and were forced to stay. Work can last up to 18 hours a day in the heat of July and August.

"The work is extremely physically damaging; it's all manual labour," said Jane Buchanan, the report's author, speaking to the Guardian from Almaty, the largest city in Kazakhstan. "People come as families, they work as entire families, and they often don't get paid for eight or nine months, until the end of the season."

Cultivating, harvesting and drying tobacco can be an unhealthy business. Exposure to pesticides and fertilisers can cause sickness, with children being particularly vulnerable. Researchers found that workers had been given little information about "green tobacco sickness" – an ailment caused by absorption of nicotine through the skin that can cause nausea, weakness and dizziness.

In a statement from its headquarters in Switzerland, Philip Morris said it was grateful to HRW for bringing the problems to its attention and pledged a series of steps, including third-party monitoring, improved training and internal controls and a requirement that farmers had written contracts with all of their workers.

"We are committed to working to prevent child labour, forced labour and other labour abuses in the tobacco supply chain," said the company, which was spun off by the US multinational Altria two years ago.

Philip Morris's tobacco supplies from Kazakhstan are used for the company's local brands in central Asia, Russia and Ukraine, rather than for international names such as Marlboro, Chesterfield or Parliament.