NATO head Jens Stoltenberg will travel to key ally Turkey this week to meet President Recep Tayyip Erdogan, one of the highest western officials to visit since a failed military coup in July, a statement said Tuesday.

The alliance gave no further details of Stoltenberg's meeting Thursday and Friday with Erdogan, who has berated the West for what he sees as its lukewarm backing and criticism of his massive crackdown on coup suspects.

Some 20,000 people have been arrested since the July 15 coup attempt and 70,000 civil servants have been fired as Erdogan purges all those he says have links to Fethullah Gulen, the US-based Muslim preacher Ankara blames for the attempted putsch.

NATO and Stoltenberg have repeatedly expressed support for the Turkish government in the coup's aftermath, insisting that the US-led alliance means what it says in supporting democracy.

At the same time, NATO has been cautious on the crackdown, calling for Ankara to ensure human that rights norms that all alliance members are supposed to adhere to are not compromised.

US Vice President Joe Biden visited Turkey late last month, the most senior western official to do so since the coup, as Washington tried to mend ties badly strained by Ankara's demand that it hand Gulen over for trial.

European Union foreign affairs chief Federica Mogherini is due to visit Turkey on Thursday and Friday as well, following a trip last week by European Parliament head Martin Schulz who has been notably critical of the crackdown.

Turkey is a key NATO ally, second only in military numbers to the United States, but it has found itself at odds with Washington over Syria where last week it launched an incursion to set up a no-fly zone on its southern border.

Saying it was attacking Islamic State jihadis as part of the US campaign against IS in Syria and Iraq, Ankara also clashed with the Kurdish rebels Washington has backed as the best fighting force against the jihadists.

In early August, NATO had to make clear that Turkey's membership of the alliance was "not in question" when Erdogan made a highly-symbolic visit to Russia to mend fences with Moscow after Turkish jets shot down a Russian fighter in November along the Syrian border.