The Cottagers had an initial approach for the Sweden international knocked back, but are refusing to give up in their pursuit of the full-back.
The 24-year-old impressed Roy Hodgson with a number of assured performances during Euro 2008 and his agent has confirmed that the Premier League side are the only club to have made an official approach.
"Fulham's first offer was rejected, but they are interested enough to have made a new and improved offer for Fredrik," Per Jonsson told Adresseavisen.
"There is a lot of interest in him, with two French clubs also in the picture.
"But it is only Fulham who have made a serious offer."