An international trade union has asked ship operators handling goods in Walmart's global supply chain to raise concerns with the company about how it treats its US workforce.

Walmart has been affected by a series of walkouts and protests by several union-supported groups seeking to highlight what they say are low pay, poor benefits and retaliatory measures against those employees who speak out.

A series of high-profile protests are now planned to highlight "Black Friday" this week, which is the busiest single shopping day in the US calendar.

Organisers behind the OUR Walmart and Making Change at Walmart groups say up to 1,000 actions are planned and several walkouts have already happened.

Now the International Transport Workers' Federation (ITF) has written to shipping owners and ship captains who carry Walmart goods and asked them to contact the gigantic global company and express support for the protesting workers. "Walmart workers taking industrial action know that their jobs are at risk. The least we can do to help is use our expertise at sea and relations with the shipping industry to back them in any way we can."

ITF acting general secretary Steve Cotton told the Guardian: "We're talking to captains and the ship operators moving Walmart goods, and asking them to register their concerns with the company about its treatment of staff – and the impact that could have on trade."

The ITF is a global union federation representing around four and a half million transport workers worldwide.

In recent months, parts of Walmart's outsourced warehouse supply chain in the US have been hit by strikes and demonstrations. Walmart has accused unions of seeking to cause trouble and organise its workforce. It has said previously that only a tiny minority of its 1.3 million US staff are joining the protests and has defended its wages and benefits as offering good jobs to hundreds of thousands of Americans.

But the protests do appear to have rattled the firm. Walmart has filed a complaint with the labor board asserting that OUR Walmart's protests violate federal law that prevents 30 days of picketing when a union is seeking recognition. Walmart says the protests fit that description and are actually sponsored by the United Food and Commercial Workers International Union. It has sought an injunction to prohibit the protests. Both OUR Walmart and the UFCW deny those allegations and say that they are not seeking union recognition.

However, Walmart spokesman Steve Restivo said: "There are only a handful of associates at a handful of stores who are participating in these UFCW publicity stunts. Most of the folks who are turning out aren't Walmart associates but instead union representatives and members. An overwhelming majority of our associates are excited about Black Friday and are ready to serve our customers. We're proud of the job they do not only during this busy holiday time but also throughout the year."

OUR Walmart has also filed complaints alleging that public statements made by Walmart executives have amounted to a threat to protesting employees. Walmart spokesman David Tovar this week warned on CBS Evening News of the possible consequences for employees walking off their scheduled shifts. "If associates are scheduled to work on Black Friday, we expect them to show up and to do their job. And if they don't, depending on the circumstances, there could be consequences," he said.

That statement angered some OUR Walmart members. "Some of my co-workers are afraid, but this kind of intimidation by Walmart management is an example of why we are going on strike. I know my rights and I'm not afraid to protest," said Dan Hindman, a California Walmart worker and member of OUR Walmart.

Those protests look set to go ahead and range from walkouts to leafleting of shoppers as they crowd into stores in the hunt for bargains to stunts like "flash mobs" and other events.

They are currently planned in various cities in states that include California, Illinois, Texas, Maryland, Louisiana, Florida, Oklahoma, Mississippi and Wisconsin.