Carlo Ancelotti's side have four days to reach an agreement with Benfica before the transfer window closes.
Chelsea are thought to have had a £17m bid for Luiz - a centre-half who can also play as a right or left-back, rejected at the end of December.
The Benfica statement read: "We have received an offer from Chelsea Football Club for the acquisition of the sporting and economic rights of David Luiz.
I am so sick of this saga. I don't care whether we sign him. Defenders are about the most boring transfers you can make anyway
"A negotiation process between both parties is ongoing and the market will be informed when it is completed."
Ancelotti said on Monday: "He is the only signing we are trying to make but if it's not possible, nothing changes. He's a very good young player but we have not agreed anything."
Chelsea were confident enough of concluding the transfer this week to apply for a work permit, which was granted on Wednesday, although Luiz played the whole of Benfica's Portuguese Cup win at Rio Ave on the same day.
The dispute reportedly centres on the precise make-up of the deal, with Benfica unwilling to entertain the fee being paid in instalments.
They are also said to be unhappy with the idea of a player-plus-cash deal, with Nemanja Matic - currently on loan at Vitesse Arnhem - having been mentioned as a possible makeweight.
Luiz, whose contract at Benfica expires in 2015, is widely regarded as one of the most promising defenders in Europe.
He arrived in Lisbon from Brazil in 2007 and was a key member of the side that last season clinched the Portuguese title for the first time in five years, missing only one of 30 league matches.
His performances earned him a call-up to the Brazil squad for August's friendly against the United States.
Were he to join Chelsea, he would link up with fellow Brazilians, defender Alex and midfielder Ramires.
Meanwhile, Jacob Mellis today became the latest Chelsea youngster to commit his future to Stamford Bridge after signing a new contract that will keep him at the club until 2013.
The 20-year-old midfielder, who made his first-team debut as a last-minute substitute in November's Champions League win against MSK Zilina, has represented England Under-19s and appeared 15 times on loan at Southampton in the first half of last season.
Source: BBC Sport