Borussia Dortmund sign Sweden striker Alexander Isak from AIK

Borussia Dortmund have completed the transfer of 17-year-old striker Alexander Isak from AIK Solna.

The Sweden international joins BVB for a transfer fee which a source told ESPN FC is €9.5 million. He has signed what the club described in a statement as a long-term contract, but the length of his deal still has to be determined by FIFA, due to his age.

The attacker had strongly been linked with a switch to Real Madrid last week, but opted to join Borussia Dortmund instead after the Bundesliga club made a late move for the Sweden international.

"Alexander Isak is a high-quality attacking talent and a lot of European top clubs hoped to sign," BVB sporting director Michael Zorc said, and the announcement on the club's official homepage added that Isak signed a long-term contract. "We are delighted he opted for Borussia Dortmund. BVB and the player are absolutely convinced that this transfer is one with a big perspective."

At the weekend, a source close to Isak told ESPN FC that the transfer will "set a new landmark in Swedish domestic football" and top the transfer fee Malmo received for Zlatan Ibrahimovic from Ajax in 2001.

Isak, who has been compared with Manchester United forward Ibrahimovic in the past, believes Thomas Tuchel's men to be a "smarter choice" than Real Madrid, the source told ESPN FC.

Isak became the youngest player to ever score for Sweden when he struck in the 6-0 win against Slovakia on Jan. 12.

The son of Eritrean parents has scored 10 goals in 24 Allsvenskan appearances for his hometown club AIK Solna last season, and was monitored by scouts from Europe's biggest clubs which also included Chelsea.

Isak is set to replace Adrian Ramos, 31, in Dortmund's squad. The Colombian attacker is on the verge of completing a €12m transfer to CSL club Chongqing Lifan from where he will be immediately loaned out to La Liga strugglers Granada, who are under the same ownership.