Turkish President Recep Tayyip Erdogan called Saturday for a peaceful resolution of Ukraine's tensions with Russia based on respect of its territorial integrity.

Flanked by his Ukrainian counterpart Volodymyr Zelensky, Erdogan told reporters in Istanbul that Turkey aims "to ensure the Black Sea remains a sea of peace and cooperation" as US warships were due to sail through Turkey's Bosphorus.

"We don't want an increase in tensions in our common region," Erdogan said. "We believe that the current crisis must be settled by peaceful means on the basis of international law and respect for Ukraine's territorial integrity," the Turkish leader added.

Ukrainian and US officials have expressed concern in recent days over the arrival of thousands of Russian soldiers and materiel near eastern Ukraine where clashes have increased between pro-Russian separatists and Kiev's government forces in a conflict now several years old.

Turkey announced Friday the United States will soon send two warships through the Bosphorus to the Black Sea, which borders both Russia and Ukraine. US Navy ships routinely operate in the region in support of Ukraine.

Erdogan also said Turkey was ready to provide all forms of support aimed at settling the conflict and that it would refuse to recognise Russia's 2015 annexation of Crimea, which was part of Ukraine.

Zelensky said: "Turkey's support for re-establishing our territorial sovereignty is essential." Turkey said the two US warships would transit the Bosphorus in line with the Montreux Convention. 

The 1936 treaty guarantees free navigation for civilian ships through the Turkish strait. But states that do not share a Black Sea coastline must give two weeks notice before sending warships into the area and then keep them there for no more than 21 days.

In a phone call on Friday, Russian President Vladimir Putin urged his Turkish counterpart to protect the treaty at a time when questions have risen about the impact of a planned canal. Analysts said the possible suspension of the treaty could facilitate access to the Black Sea of warships from non-coastal countries like the United States and other NATO member states, which Moscow frowns upon. Turkey is a NATO member.