Scarlett Johansson has ended her role as global ambassador for Oxfam, with the aid group saying it deemed the position "incompatible" with her promotion of an Israeli firm that has a factory in the occupied West Bank.

The 29-year-old US actress appears in an advertisement for SodaStream, which is due to air during the US Super Bowl on Sunday.

It has already been seen more than 4.5 million times on the YouTube video-sharing website.

The Hollywood star has worked on behalf of Oxfam since 2005 but the British-based aid agency said she had stepped down due to her role with SodaStream, which manufactures products for people to make fizzy drinks at home.

"Oxfam has accepted Scarlett Johansson's decision to step down after eight years as a global ambassador and we are grateful for her many contributions," the charity said in a statement.

"While Oxfam respects the independence of our ambassadors, Ms Johansson's role promoting the company SodaStream is incompatible with her role as an Oxfam global ambassador.

"Oxfam believes that businesses such as SodaStream, that operate in settlements, further the ongoing poverty and denial of rights of the Palestinian communities that we work to support.

"Oxfam is opposed to all trade from Israeli settlements, which are illegal under international law."

Super Bowl half-time ads usually see companies go all-out with expensive TV spots and are enjoyed by US viewers as an key part of the whole event.

Sunday's Super Bowl is not only the United States' biggest sporting event, it is one of the biggest advertising events on the planet, when a record $4 million (three million euros) buys 30 seconds of airtime.

More than 110 million television viewers in the US are projected to watch Sunday's game -- about a third of the population -- when the Denver Broncos take on the Seattle Seahawks at MetLife Stadium, just west of New York.

The SodaStream ad had been banned by Fox for taking a swipe at Pepsi and Coca-Cola.

Johansson became an Oxfam global ambassador in 2007, helping to bring media attention to the impact of natural disasters and raise funds to fight poverty and save lives.

She went to India and Sri Lanka to meet survivors of the Indian Ocean tsunami and in 2011 went to a Somali refugee camp in Kenya to see the impact of the severe drought.

Recently she made short video urging people to donate to Oxfam's Super Typhoon Haiyan appeal.