President Volodymyr Zelensky hailed "brave" Ukraine on the 500th day of Russia's invasion on Saturday, as the war's toll mounted with six deaths reported in Russian rocket fire.

Zelensky published on social media an undated video clip of a visit to Snake Island in the Black Sea -- a symbol of Ukraine's defiance against Russia.

"Today we are on Snake Island, which will never be conquered by the occupiers, like the whole of Ukraine, because we are the country of the brave," he said.

"I want to thank from here, from this place of victory, each of our soldiers for these 500 days," he said in the video, which showed him arriving on the island by boat and leaving flowers.

Finishing a trip to Turkey on Saturday, Zelensky prayed for the victims of the war alongside Patriarch Bartholomew, the leader of the world's Orthodox, after a regional tour to drum up support ahead of a NATO summit next week.

The UN has documented 9,000 civilian deaths since the start of the war on February 24, 2022, including 500 children, although it estimates the real toll could be significantly higher.

That toll went up again on Saturday as authorities in the eastern Donetsk region said six people were killed by Russian rocket fire in the town of Lyman.

"At around 10:00 am, the Russians struck the town with multiple rocket launchers," Donetsk regional governor Pavlo Kyrylenko said on social media, adding: "At least six people were killed and five injured."

- 'Grim milestone' -

Noel Calhoun, deputy head of the UN's Human Rights Monitoring Mission in Ukraine, said the 500th day of the conflict marked "another grim milestone in the war that continues to exact a horrific toll on Ukraine's civilians".

In Turkey, Zelensky secured backing for his country's NATO aspirations after winning a US pledge for cluster munitions that could inflict significant damage on Russian forces on the battlefield.

The US decision to deliver the weapons -- banned across a large part of the world but not in Russia or Ukraine -- dramatically ups the stakes in the war.

US President Joe Biden admitted that supplying Ukraine with weapons that are capable of covering several football fields with hundreds of multiple small explosives was "a difficult decision".

Humanitarian groups strongly condemned the decision to supply cluster munitions, which can go undetonated and potentially endanger civilians for years to come.

Various international efforts to mediate in the conflict have so far failed.

Turkish President Recep Tayyip Erdogan has tried to portray himself as a neutral mediator, substantially boosting wartime trade with Russia while supplying Ukraine with drones and other weapons that helped keep Kremlin forces from seizing Kyiv in the first weeks of war.

But while reaffirming his longstanding call for both sides to enter peace talks, Erdogan risked drawing Russia's ire by delivering unequivocal support for Ukraine's NATO aspiration.

"There is no doubt that Ukraine deserves membership of NATO," Erdogan told reporters in Istanbul.

Erdogan also said he would meet Russian President Vladimir Putin next month on what would be the Kremlin chief's first visit to Turkey since the invasion.

The Turkish leader said he and Putin will discuss possible prisoner swaps, as well as a possible extension of a deal brokered last year under which Ukraine was able to ship grain to the global market.

The agreement will expire on July 17 unless Russia agrees to its renewal.

Asked about a possible meeting with Erdogan, Kremlin spokesman Dmitry Peskov was quoted by Russian news agencies on Saturday as saying: "Contact is possible. There are no fixed dates yet."

- Symbol of defiance -

Zelensky meanwhile vowed defiance in the video of him visiting Snake Island, which was captured by Russian forces shortly after the start of the invasion.

A radio exchange went viral in which Ukrainian soldiers told the crew of Russia's attacking warship demanding their surrender to "go fuck yourself".

The Ukrainian soldiers were taken prisoner but later exchanged for Russian captives.

The recording of this verbal exchange has gone around the world and served as a theme for the Ukrainian resistance, even appearing on placards during support rallies abroad and on stamps.

The Russian ship involved, the Moskva, sank in the Black Sea in April 2022 following what Moscow said was an explosion on board. 

Ukraine said it had hit the warship with missiles.

Ukrainian forces recaptured the island in June last year.

bur/imm