Oxfam is under mounting pressure from campaigners opposed to settlements to sever ties with Scarlett Johansson after she agreed to star in an advertising campaign for a fizzy drinks company with a factory in an Israeli settlement in the West Bank.

Activists in the campaign for boycott, divestment and sanctions against Israel have seized on Johansson's appearance in an advert for SodaStream scheduled to air during half-time at this Sunday's US Super Bowl, the advertising slot with the largest audience on American television.

Johansson has been an Oxfam goodwill ambassador since 2005. The campaigners say her position is untenable.

"Oxfam is a human rights organisation. They cannot maintain an ambassador if they are involved in a complicit Israeli company built in a settlement. They can't keep both. You can't maintain something and its contradiction," the boycott campaign founder Omar Barghouti told the Guardian.

"Activists everywhere, particularly in the US, UK and Palestine as well as affiliated organisations and major Palestinian figures are writing, tweeting, or calling Oxfam to dismiss Johansson as ambassador, because of her conscious decision to lend her name to whitewashing Israeli occupation and apartheid," Barghouti said.

This week, Johansson issued a statement defending her decision to promote the home carbonated drink company, which employs hundreds of Palestinian workers at its plant in the settlement of Ma'ale Adumim east of Jerusalem.

"SodaStream is a company that is not only committed to the environment but to building a bridge to peace between Israel and Palestine, supporting neighbours working alongside each other, receiving equal pay, equal benefits and equal rights. That is what is happening in their Ma'ale Adumim factory every working day. As part of my efforts as an ambassador for Oxfam, I have witnessed firsthand that progress is made when communities join together and work alongside one another and feel proud of the outcome of that work in the quality of their product and work environment, in the pay they bring home to their families and in the benefits they equally receive," Johansson said, adding that consumers should make an "educated choice" about whose products to use.

Oxfam faced a similar problem in 2009 when the Sex and the City star Kristin Davis signed up with Ahava, an Israeli cosmetics company that also manufactures in the West Bank. In that case, Oxfam did not drop Davis and she ended her association with Ahava soon afterwards.

An Oxfam spokesperson told the Guardian that the charity was "considering the implications" of Johansson's statement.

"Oxfam respects the independence of our ambassadors. However Oxfam believes that businesses 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," said the spokesman.

But boycott campaigners indicated that they would push Oxfam for a decision. Barghouti and his colleagues have notched up a series of recent victories in US academic bodies and say they were instrumental in the loss of billions of pounds worth of local council contracts in the UK and Europe for the French company Veolia as punishment for its involvement in a light rail network in Jerusalem.

"Oxfam will lose its credibility in Palestinian civil society, and far beyond, if it fails to act on its core values and principles by ending its links with Johansson. We shall continue our pressure campaign against it in this case, to expose what would be accurately described as a textbook case of double standards," said Barghouti.

The SodaStream chief executive, Daniel Birnbaum, told the Forward he would never have established a production plant in a settlement if he could turn the clock back, saying its location had turned out to be "a pain in the ass". He inherited the factory when he became CEO in 2007. Birnbaum said he would not bow to pressure to close it out of loyalty to his 500 Palestinian employees, who would be unable to work at a new larger plant under construction inside Israel. "We will not throw our employees under the bus to promote anyone's political agenda," he said, adding that he "just can't see how it would help the cause of the Palestinians if we fired them".

• This article was amended on 3 February 2014. In a sub-heading and in the body of the text campaigners seeking to pressure Oxfam to sever ties with Scarlett Johansson were described as "anti-Israel". To clarify: the campaigners are opposed to settlements.