Turkey has not approved the closure of the Turkish straits to Russian warships, a Turkish official with direct knowledge of the matter told Deutsche Presse-Agentur (dpa).

Turkish President Recep Tayyip Erdoğan did not tell Ukraine's President Volodymyr Zelenskyy that Turkey is closing or has closed Turkish straits to Russian warship access.

The statement follows a phone call between the two leaders Saturday.

"The ban on the passage of Russian warships to the Black Sea and significant military and humanitarian support for Ukraine are extremely important today. The people of Ukraine will never forget that!" Zelenskyy tweeted earlier on Saturday regarding the call with Erdoğan.

Turkey is still assessing the issue and Zelenskyy's remarks "only reflect his expectations," the official added.

Erdoğan told Zelenskyy that Turkey is working to secure an "immediate ceasefire" and offered his condolences over deaths in Russian attacks, the Turkish presidency said on Twitter.

The office did not mention any reference to the Black Sea.

Under the 1936 Montreux Convention, Turkey has control over the Bosporus and Dardanelles straits connecting the Black Sea and the Mediterranean. While trade ships may pass the strait freely in peacetime, battleships face certain restrictions.

Russia often uses the Turkish straits to maneuver ships between the Mediterranean Sea and the Black Sea.

On Friday, NATO Secretary-General Jens Stoltenberg said the Bosporus was not among the topics discussed at a leaders' summit.

Russian ships would still be able to pass the straits even if Turkey closes them, Turkish Foreign Minister Mevlüt Çavuşoğlu said earlier this week.

Kyiv's ambassador to Ankara, Vasly Bodnar, on Thursday officially requested that Turkey close the straits to Russian ships.

Turkey has good economic and political relations with both Russia and Ukraine, and Erdoğan earlier said Ankara did not want to alienate either state.

Closing the straits "would be a first in the post-WWII era," but it can not be interpreted as NATO going to war at all, Sinan Ülgen, the chairperson of Istanbul-based think tank EDAM, told dpa on Saturday.

"Because this would not be a decision to sanction Russia. This would be Turkey's commitment and obligation under the treaty, in case of a war in the Black Sea which Turkey is not a party to."

Russian ships which belong to the Black Sea fleet will nonetheless be able to return to their ports according to Montreux deal, Ülgen said, adding he believes Russia "will make use of the exception."