Dempsey has once again been a stand-out performer for Fulham this season, scoring 12 Premier League goals from his withdrawn attacker role.
Recently his partnership with Pavel Pogrebnyak has sparked the Cottagers to three straight wins, lifting them to eighth in the Premier League.
The USA international currently has 17 months left on his contract, taking him to the end of next season, but Fulham want to add at least another year to that deal, and Jol admits negotiations are underway.
"We are talking to him," Jol said. "At any club, you want to keep your best players so that is what we try hard to do. If Clint Dempsey wasn't here, we wouldn't be in this position because we have had to rely on him a lot of times to score goals.
"He is here for at least another year so that is not a big problem. The problem is that we want to keep him here for longer. But the difference is not very big.
"He has 17 months left and we want to keep him here for the next three years. I hope the four or five players we are talking to will stay. I am confident they will. They realise it is a very good club."