Amazon employees say the company is threatening to fire workers for publicly pushing their employer to do more to combat climate change.

Amazon Employees For Climate Justice, a climate change advocacy group founded by Amazon workers, said Thursday that the company sent letters to its members telling them that they could be fired if they continued to speak to the press. At least four of the group's members were threatened with firing, a spokesperson said.

"This is not the time to shoot the messengers," Amazon employee Maren Costa said in a prepared statement. "This is not the time to silence those who are speaking out."

We must be able to speak up. Here is our press response to Amazon’s intimidation tactics. 3/ pic.twitter.com/7DqhCw09Yf — Amazon Employees For Climate Justice (@AMZNforClimate) January 2, 2020

Amazon, which relies on fossil fuels to power the planes, trucks and vans that ship packages all over the world, has an enormous carbon footprint, last year releasing the equivalent of 44.4 million metric tons of CO2—as much as Hong Kong (but less than Walmart). Amazon workers have been increasingly vocal about calling out their employer's practices.

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Last year, more than 8,000 staffers signed an open letter to CEO and founder Jeff Bezos, demanding that Amazon cut its carbon emissions, end its use of fossil fuels and stop its work with oil companies that use Amazon's technology to locate fossil fuel deposits. Some workers also pushed for a shareholder resolution requiring Amazon to reduce its carbon footprint. While the resolution failed, Amazon revealed details on its carbon emissions and pledged to cut it shortly after.

The online shopping giant said Thursday that employees can suggest improvements on how it operates within the company.

"Our policy regarding external communications is not new and we believe is similar to other large companies," Amazon said.

The advocacy group disagreed and said company officials changed the communications policy in September, one day after workers said they would participate in the global climate strike. Employees are now required to get approval before speaking about Amazon publicly.

"Amazon's newly updated communications policy is having a chilling effect on workers who have the backbone to speak out and challenge Amazon to do better," said Victoria Liang, an Amazon software engineer and climate change group member. "This policy is aimed at silencing discussion around publicly available information. It has nothing to do with protecting confidential data, which is covered by a completely different set of policies."

In one email that was reviewed by CBS MoneyWatch, an Amazon official told a worker that speaking to the Washington Post about the company's climate change actions violated Amazon's external communications policy, and warned the worker not to do it again.

"Future violations of the policy may result in formal corrective action, up to and including termination of your employment with Amazon," the email says.

Major tech companies, including Google, have been grappling with how to deal with employees who increasingly speak out about corporate policies and practices.

Amazon recently vowed to source all of its energy from renewable sources and to achieve carbon neutrality by 2040.

Carbon dioxide and other greenhouse gases cause global warming, which can lead to rising sea levels and more destructive meteorological events.

CBS News' Khristopher Brooks and Irina Ivanova contributed reporting.