Chelsea are stepping up their bid for Benfica centre-back David Luiz, but are also looking at Danish star Simon Kjaer as a back-up if they cannot persuade the Brazilian to move.

GettyImagesDavid Luiz and Simon Kjaer are high on the list.

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Carlo Ancelotti has earmarked a top quality centre-back as his main priority for the January transfer window, as the injury problems to skipper John Terry and Alex, alongside the sale of Ricardo Carvalho to Real Madrid in the summer, have left Chelsea vulnerable at the back.

Benfica blocked Luiz's move to Stamford Bridge last summer demanding the full £38 million stipulated in his buyout clause.

The Portuguese club have since softened their stance, but are still holding out for a minimum of £25 million, while Chelsea have so far made two bids - one of £17 million and the second of £21 million plus right-back Paulo Ferreira - but neither have been accepted.

Negotiations are still ongoing, but Chelsea have also been tracking the progress of Wolfsburg defender Kjaer.

A leading agent close to the negotiations told ESPNsoccernet: "Chelsea desperately want a centre-half, and while they have been trying to buy Luiz for some time, they have also been very impressed with Kjaer.

"They have a list of about five centre halves, and there is little doubt they will buy at least one of them this January transfer window."