US labour officials have found that Amazon retaliated illegally when it fired two employees who had spoke out about the firm's environmental practices and warehouse safety.

The National Labor Relations Board was investigating a complaint brought by Emily Cunningham and Maren Costa, who had helped organise the group Amazon Employees for Climate Justice.

They were fired last April.

Amazon said it disagreed with the finding.

"We support every employee's right to criticise their employer's working conditions, but that does not come with blanket immunity against our internal policies, all of which are lawful," an Amazon spokesman said on Monday.

"We terminated these employees not for talking publicly about working conditions, safety, or sustainability, but rather, for repeatedly violating internal policies."

The National Labour Relations Board (NLRB) will file its own complaint if the firm does not settle the case.

"It's a moral victory and really shows that we are on the right side of history and the right side of the law," Ms Cunningham told the New York Times, which first reported the conclusions of the NLRB's investigation of the incident.

In recent months, the labour agency has received more than a dozen complaints alleging unfair retaliation by Amazon by staff, according to online records.

Officials are reviewing the cases to see if they merit a consolidated investigation into the firm's practices.

Amazon has a reputation for reacting aggressively to labour activism and barring staff from speaking publicly about the company.

The issue has gained new urgency amid tensions brought on by the pandemic, which brought a surge in business and profit to Amazon, while raising new health risks for warehouse workers.

The company has been hit by strikes and other actions in Europe. In the US, it is facing its first formal union drive since 2014.

New York's attorney general is also investigating whether it illegally retaliated against warehouse workers in the state who organised protests during the pandemic.

The dispute with Ms Cunningham and Ms Costa, designers at the company, pre-dates the virus.

In 2019, they were part of a group that helped organise a walkout at the firm's Seattle headquarters to pressure the company to improve its environmental practices. Employees also pushed the firm to develop a policy to tackle climate change at its annual board meeting for investors.

That same year, Amazon boss Jeff Bezos said Amazon would aim to be powered by renewable energy by 2030.

Amazon Employees for Climate Justice hailed the NLRB finding and said its current focus is pushing Amazon to address its "disproportionate environmental impact in communities of colour".

"Amazon tries to stoke fear in employees that speaking out is wrong, but the NLRB's ruling confirms that it is legally right for us to speak out as employees, just as we know we are in the moral right to protect our own climate future," the group said in a statement.