Manchester United will look to step-up their efforts to sign striker Jackson Martinez - sparking fears they could be ready to let Robin van Persie leave. Martinez, who has been linked with a number of Premier League sides including Liverpool, Chelsea and Tottenham Hotspur in recent times, has a buy-out clause of £34m in his contract, which runs until 2016. The Colombian is regarded as one of the best strikers in Europe, having scored 27 goals in 45 appearances for Porto in all competitions so far this season.