UPDATE 4-Erdogan 'saddened' by Russian invasion, Ukraine urges Turkey to shut straits

* Erdogan 'sincerely saddened' by Russian invasion

* Turkish foreign ministry calls for end to fighting

* Kyiv asks Turkey to close Black Sea waterways to Russia

* Ankara will evaluate the request, Ukrainian envoy says (Adds Turkish ruling party spokesman's comments, details)

By Tuvan Gumrukcu

ANKARA, Feb 24 (Reuters) - Turkey backs Ukraine's territorial integrity and is "sincerely saddened" by Russia's invasion, President Tayyip Erdogan said on Thursday, as Kyiv's envoy to Ankara urged Turkey to close the Bosphorus straits to Russian warships.

Earlier on Thursday, Russian forces launched an invasion of Ukraine, assaulting by land, sea and air in the biggest attack by one state against another in Europe since World War Two.

Erdogan, who has forged good relations with the leaders of both Russia and Ukraine and had previously offered to mediate in the conflict, said he had spoken with Ukrainian President Volodymyr Zelenskiy by telephone.

"Turkey supports Ukraine's battle to protect its territorial integrity," Erdogan said in remarks broadcast by Turkish TV.

"We are sincerely saddened that Russia and Ukraine, both of whom we see as friendly countries and with whom we have close political, economic, and social relations, come face to face in this way."

Ukraine's ambassador to Ankara, Vasyl Bodnar, told reporters that Zelenskiy had asked Erdogan for help during their call. The ambassador did not outline Kyiv's demands, but said "financial, humanitarian, and military" support was needed.

Turkish Defence Minister Hulusi Akar later discussed the latest developments in Ukraine with his Ukrainian counterpart Oleksiy Reznikov, his ministry said.

BOSPHORUS

Earlier, Bodnar had urged NATO member Turkey to close the Bosphorus and Dardanelles straits to Russian ships.

Under a 1936 pact, Ankara has control over the straits and can limit the passage of warships during wartime or if threatened.

"Turkey will evaluate the requests and respond as soon as possible," Bodnar said after talks with Turkish Deputy Foreign Minister Sedat Onal. "We expect solidarity to be shown."

Ukraine's appeal puts Turkey in a difficult position as it shares a maritime border with Ukraine and Russia in the Black Sea and does not want to antagonise either country.

Asked about Ukraine's request, Omer Celik, the spokesman for Erdogan's ruling AK Party, said Turkey was evaluating the issue and that Ankara would make a decision with the aim of not deepening the conflict. He provided no further details.

Earlier this month, six Russian warships and a submarine transited the Dardanelles and Bosphorus straits to the Black Sea for what Moscow called naval drills near Ukraine waters.

On Thursday Russian forces landed at Ukraine's Black and Azov Sea ports as part of the invasion.

Turkey's foreign ministry called for an immediate end to Russia's military operations.

"This attack is a grave violation of international law and poses a serious threat to the security of our region and the world," the ministry said. "We call on the Russian Federation to immediately end this unjust and unlawful act."

Ambassador Bodnar urged Turkey to join other countries in imposing sanctions on Russia, something Ankara has so far opposed as it tries to balance its NATO commitments with its strong trade and energy ties to Russia.

Celik said Turkey was not considering any sactions against Russia at present.

On Thursday, Turkey advised its citizens in Ukraine to stay at home or in a safe place and avoid traveling, after airlines cancelled flights due to the closure of Ukraine's air space.

(Additional reporting by Ece Toksabay and Ali Kucukgocmen; Editing by Daren Butler and Gareth Jones)