Turkey's purchase of Russian S-400 surface-to-air missile systems is a done deal and cannot be canceled, Turkish Foreign Minister Mevlut Cavusoglu said on Tuesday.

Ankara needs further defense procurement that could be bought from the United States, Cavusoglu added.

Turkey's purchase of the S-400 systems, which are not compatible with NATO defenses, has unnerved the United States and the alliance's member countries, which are already wary of Russia's presence in the Middle East.

"The current deal is a done deal, I cannot cancel it," Cavusoglu told reporters after meeting with American counterpart Mike Pompeo. "But I need more ... and I prefer to buy from my allies," he added.

Turkey and Russia have also strengthened energy cooperation.

The two sides on Monday completed the offshore section of the TurkStream pipeline which bypasses Ukraine and connects the two countries.

Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin attended the inauguration in Istanbul, emphasizing the benefits of the project for the two countries as well as the region. 

TurkStream comprises two lines, each with the capacity to carry 15.75 billion cubic meters of natural gas per year. The first line will be used to transit gas to Turkish consumers, while the second line is expected to deliver gas to southern and southeastern Europe. 

Russia expects to finish the onshore section by the end of 2019. 

(With inputs from Reuters)