US actress Scarlett Johansson has quit as global ambassador for Oxfam after the aid group deemed the role "incompatible" with her promotion of an Israeli firm that has a factory in the occupied West Bank. Johansson, 29, appears in an advertisement for home drinks carbonation firm 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 for Oxfam since 2005 but the British-based aid agency said she had stepped down due to her role with SodaStream. "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.