<p class="big">David Sullivan admits Birmingham City hold an interest in former Middlesbrough team-mates Jimmy Floyd Hasselbaink and Mark Viduka. </p> Following Blues' promotion back into the Premiership, the club are looking to strengthen the squad to prevent relegation again. </p> Attack is a key area to improve and co-owner Sullivan concedes Hasselbaink, who was released by Charlton Athletic earlier in the week, is under consideration. </p> "He's a possibility," Sullivan told the Birmingham Mail about the veteran Dutch striker. </p> "We are ruling nothing out, but neither are we ruling anything in. </p> "Definitely, we need a striker who will score goals in the Premier League. Sheffield United got relegated because they were one goal worse off than Fulham and Wigan." </p> Regarding a potential deal for Viduka, Sullivan accepts it could be more difficult to entice the Australian to St Andrews. </p> Viduka's contract at Middlesbrough expires shortly and he has yet to make a decision on his future, with several clubs monitoring his situation. </p> Birmingham are one of those and, while acknowledging the ex-Leeds United forward has his faults, Sullivan admitted an interest. </p> He added: "The trouble is, eight clubs are chasing him. </p> "Viduka's 31 and goes missing in some games - but he scores goals." </p>