US diplomats have cast doubts on the reliability of NATO ally Turkey, portraying its leadership as divided and permeated by Islamists, according to the German Der Spiegel's website.

Der Spiegel, citing US diplomatic documents released on Sunday by whistle-blowing website WikiLeaks, said advisers to Prime Minister Tayyip Erdogan were described as having "little understanding of politics beyond Ankara".

Erdogan has introduced sweeping liberal economic reforms in pursuit of European Union membership since his AK Party was first elected by a landslide in 2002. He denies accusations by secularists that he harbours secret Islamist ambitions.

Der Spiegelcited a cable as saying Erdogan had surrounded himself with an "iron ring of sycophantic (but contemptuous) advisors".

Turkey has traditionally had close relations with Washington, but ties have been strained of late, partly as a result of Ankara's falling out with Israel over its invasion of Gaza.

"The leaked diplomatic cables reveal that US diplomats are sceptical about Turkey's dependability as a partner ... The leadership in Ankara is depicted as divided and permeated by Islamists."

Der Spiegel's website gave few details or direct quotations from the cables but said its magazine would provide a broader account today.