Ahead of an EU summit today, Merkel said that Turkey should be praised in what it has done, while Europe has not "covered itself with glory in how, as a union of 28 member states with 500 million citizens, it has struggled with fairly sharing the burden".
The summit comes after a deal was agreed in principle last week, in which the EU will give benefits to Turkey in exchange for the the country taking back all economic migrants reaching the Greek islands in an effort to close off the Balkans route.
In exchange for each Syrian migrant returned, Turkey wants the EU to accept one refugee, as well as incentivise the country by offering extra funding and early access to European visas.
However, European Council President Donald Tusk has warned that there are a "catalogue of issues" in the way of such an agreement.
Turkey's potential EU membership bid has no guarantee of being processed more rapidly. The EU has merely offered further negotiations.
Meanwhile, the Prime Minister is to warn that there could be a renewed wave of north African migrants arriving on the shores of Europe this summer.
Cameron is expected to use the EU summit to urge that attention is also focussed to the north African route as action is taken to make it harder for migrants to cross into Europe in the eastern Mediterranean.