Mr Westerwelle (left) signalled a softer German line towards Turkey
Germany has pledged not to block Turkey's bid to join the EU, but has urged it to press on with reforms.
"What the EU and Turkey have agreed stands. And that applies to this German government too," German Foreign Minister Guido Westerwelle said.
Speaking in the Turkish capital Ankara, he called Turkey's negotiations "open-ended". Progress has been slow since Turkey began talks with the EU in 2005.
Germany's Chancellor Angela Merkel does not back full EU membership for Turkey.
She has spoken of Turkey getting a "privileged partnership" with the EU - less than full membership.
Within the ruling German coalition, Mr Westerwelle's liberal Free Democrats are seen as more open to Ankara's ambitions than Mrs Merkel's conservative Christian Democrats, correspondents say.
He is in Turkey for his first visit since the election last October that returned Mrs Merkel to office.
The BBC's Jonathan Head in Istanbul says the "privileged partnership" idea has been widely condemned in Turkey as an insult - and it has led to tense relations with France and Germany, the two countries that proposed the idea.
Turkey's accession process has become bogged down, with the divided island of Cyprus remaining the biggest obstacle.
Talks to reunite the breakaway Turkish-controlled north and Greek Cypriot south have stalled. The Republic of Cyprus is internationally recognised and already an EU member - unlike the breakaway north.
Turkey's refusal to let Cypriot ships use its ports is a violation of existing treaties with the EU. The Cypriot government has also been able to block the opening of a number of chapters in the Turkish-EU membership talks.
Mr Westerwelle urged Turkey to speed up the reforms it needs to make to meet EU membership criteria, including religious and political freedom, judicial impartiality and treatment of the Kurdish minority.
And he pointed out that however many years it takes, eventual EU membership cannot be guaranteed.