Both players have a year remaining on their existing deals but the Scottish Premiership club were keen to secure their services for the longer term.
Ciftci, 23, has scored 31 goals in 81 appearances for United attracting the interest of the Turkey U21s, having previously represented his country at international youth level.
Souttar, 18, is a product of the highly-successful Tannadice youth academy system and is a current Scotland U19, already appearing 51 times for United.
A spokesman for the club said: “Both players have proved their worth to the team and the club as a whole and merit the extension offer.
“We believe that Dundee United remains the best club for both players to develop and as such have made enhanced longer-term offers to the players that reflect their value to Dundee United.”