Johann Berg Gudmundsson and Birkir Bjarnason gave Iceland a two-goal cushion heading into half-time and things got even better for Heimir Hallgrimsson's team four minutes into the second half when Kari Arnason extended the lead.
The defeat ended Turkey's hopes of qualifying but Iceland moved two points clear at the top of the table heading into the final round of matches after Croatia could only draw at home to Finland.
A stoppage-time equaliser from Pyry Soiri left qualification out of the hands of Croatia, who led through Mario Mandzukic's 57th-minute tap-in.
Croatia face a tough last game away to Ukraine, who can still qualify themselves after a Leart Paqarada own goal and a Andriy Yarmolenko effort saw them win 2-0 in Kosovo to join Croatia on 17 points.
Spain have qualified after winning 3-0 at home to Albania, as Italy could only draw with FYR Macedonia.
The 2010 world champions scored all their goals in the first 26 minutes, with Rodrigo, Isco and Thiago Alcantara finding the back of the net.
Their position at the top of Group G was guaranteed after Italy were held to a surprise 1-1 draw at home by FYR Macedonia.
Giorgio Chiellini poked Italy in front just before half-time but Aleksandar Trajkovski, who plays his football in Serie B with Palermo, raced clear to equalise after 77 minutes.
The point was enough for Italy to make sure they finished second in the group.
The other match in that group saw Israel win 1-0 in Liechtenstein thanks to a first-half goal from Eitan Tibi.
Serbia missed the chance to secure their place in Russia after they were beaten 3-2 in Austria.
Luka Milivojevic put the Group D leaders in front but Austria, who are unable to qualify, turned the game on its head with goals from Guido Burgstaller and Marko Arnautovic. Nemanja Matic equalised with seven minutes remaining but Louis Schaub struck in the 89th minute to win it for the hosts.
Serbia should still top the group and secure automatic qualification as they host Georgia in their final game.
The race to finish second, though, will go down to the wire with a showdown between Wales and the Republic of Ireland in Cardiff.
Wales kept alive their hopes of qualification, and are now just a point behind Serbia, after Tom Lawrence's strike gave them a 1-0 win in Georgia.
Republic of Ireland are just a point further back after first-half goals from Daryl Murphy saw them defeat Moldova 2-0.
Republic of Ireland must beat Wales on Monday evening (Tuesday morning AEDT) to finish second in the group while a draw will be enough for Chris Coleman's men to go into the play-offs.