Coke declared two years ago that land grabbing is unacceptable, and as a major buyer of sugar pledged to help protect land rights of local communities, including evaluating its top 16 cane sugar-sourcing countries by 2020.

In the first of those reports released earlier this month, Coke said it found no evidence of land rights abuses by five sugar mills it buys from in Guatemala.

However, those findings raised questions at a World Bank conference on Land and Poverty, given Guatemala's long and often violent history of land conflict and human rights abuses.

Karol Boudreaux, land expert at the Cloudburst Group consulting firm, said the report was too superficial, relying on the word of sugar mill operators without digging deeply into their land acquisition methods.

"How did the owners acquire title documents? Were communities consulted? Were they compensated for their land? Were women's title considered?" Boudreaux said at the World Bank session on Tuesday.

"I cannot tell from the evidence that Coke presents," she told Ed Potter, the company's director of global workplace rights, who agreed to take her advice and improve the company's review processes.

In Cambodia, Coke faced criticism for benefiting from massive land thefts orchestrated by government officials for agribusinesses, including one of Coke's major sugar suppliers, said David Pred of the human rights group Inclusive Development International.

Thousands of Cambodians were violently displaced by police and military, who stole their crops, burned their homes and bulldozed their land for sugar companies, said Pred, managing director of the California-based organisation.

"What we are talking about here is grand theft," he said, adding that Coke failed to conduct serious due diligence on its supplier.

DIFFICULT FOR COKE

Coke's Potter acknowledged that more work lies ahead for the company to live up to its 2013 pledge of zero tolerance for land grabbing. But the company is determined to "do its best and get its arms around it".

"This is hard-going, difficult work," he said.

Land tenure and legal issues are complicated, records incomplete or non-existent, and working through supply chains for 207 countries will take a long time.

In India, for example, after three years and $25 million (17 million pounds) invested, Coke has yet to obtain clear land title for half of its bottling operators there, he said.

Moreover there are few experts available to perform the high-quality, in-country checks needed of suppliers on the widening array of issues that range from slavery, child labour and land grabbing to environmental degradation, he said.

Additionally, the guidance documents issued by human rights groups are multiplying, leaving no clear standards or measurements for declaring that supply chains are "clean" - let alone helping corporate boards decide when to pull out of supplier relationships, he and other participants said.

Chris Jochnick, a director at Oxfam America, which scores the 10 biggest global food companies on social justice issues, said big corporations have a responsibility to remain engaged and use their influence on the world stage to create change around human rights issues.

The exact rules of engagement though are a work in progress.

"At what point does zero tolerance for land grabs mean you cut off the supplier versus engaging with them? We don't know yet," Jochnick said.

(Reporting by Stella Dawson, editing by Alisa Tang)

By Stella Dawson