Sheffield United have completed the signing of Senegal international defender Mamadou Seck on a free transfer from Le Havre.The 27-year-old central defender has agreed a two-and-a-half-year deal with the Premiership club. Seck has made six international appearances for Senegal, the last coming in June 2005. The Blades have also confirmed the signing of Jamaica international Luton Shelton from Swedish club Helsingborg for a fee in the region of £2million. Both clubs agreed a deal for the 21-year-old, who has agreed personal terms with the Premiership club and his transfer is now subject to receiving the appropriate clearance. Shelton is an international team-mate of Blades defender Claude Davis and has scored one goal in four appearances for Jamaica. Meanwhile, striker Neil Shipperley headed in the opposite direction after his contract was terminated by mutual consent. The 32-year-old scored 11 times last season as the Blades were promoted from the Championship. Being without a club means the former Barnsley and Crystal Palace striker is free to find a new team even after the closure of the transfer window at the end of January.