Several outlets in the U.K. and in the United States have reported that the 26-year-old is on trial, and could join the club permanently if he impresses manager Sam Allardyce.
In an email to Goal.com, a Sporting KC spokesman said "Graham is training at West Ham and will join the U.S. in California later in the week."
Zusi enjoyed a highly successful 2012 campaign, becoming a regular with the U.S. national team while finishing third in MLS MVP voting. He did sign a contract extension with Sporting KC early in 2012, so West Ham would have to produce an acceptable bid to secure the University of Maryland product's services.
Zusi was called up for the U.S. national team's January camp, which began in Los Angeles this week. Also called up was Chivas USA forward Juan Agudelo, who is currently training with West Ham alongside Zusi.