A group of Amazon employees has said the company has threatened to fire some of them for speaking out on environmental issues.

Amazon Employees for Climate Justice said the workers were told they were in violation of company policies.

It comes after employees joined calls for the e-commerce giant to do more to tackle climate change.

The company said its policy on employees making public comments is not new and covers all of its workers.

In a Twitter post, the group said some employees had been contacted by Amazon's legal and human resources teams and questioned about public comments they had made.

The statement went on to say: "Some workers then received follow-up emails threatening termination if they continue to speak about Amazon's business."