President Volodymyr Zelensky on Saturday secured Turkey's crucial backing for Ukraine's NATO aspirations after winning a US pledge for cluster munitions that could inflict massive damage on Russian forces on the battlefield.

Washington's decision to deliver the controversial weapons -- banned across a large part of the world but not in Russia or Ukraine -- dramatically ups the stakes in the war, which entered its 500th day Saturday.

Zelensky has been travelling across Europe trying to secure bigger and better weapons for his outmatched army, which has launched a long-awaited counteroffensive that is progressing less swiftly than Ukraine's allies had hoped.

He called the latest US arms package "timely, broad and much-needed", tweeting that it "will provide new tools for the de-occupation of our land".

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".

"And by the way, I discussed this with our allies," Biden told CNN. "The Ukrainians are running out of ammunition."

"They either have the weapons to stop the Russians now -- keep them from stopping the Ukrainian offensive through these areas -- or they don't. And I think they needed them," he said.

US National Security Advisor Jake Sullivan explained the decision by saying there is "a massive risk of civilian harm if Russian troops and tanks roll over Ukrainian positions and take more Ukrainian territory and subjugate more Ukrainian civilians because Ukraine does not have enough artillery."

Kyiv "has provided written assurances that it is going to use these in a very careful way," he said, noting that Ukraine's government "has every incentive to minimize risk to civilians, because it's their citizens."

Ukraine pledged that it would not use the rounds in civilian-populated areas, and would record where they use them to assist demining efforts after the war, added US Undersecretary of Defense Colin Kahl.

'Keep them in the fight'

The United States would also not provide cluster munitions with a "dud rate" of more than 2.35 percent, Kahl said, contrasting this with 30-40 per cent rate of such weapons used by Russia in Ukraine.

Referring to the Ukrainian counteroffensive, he said "it's been hard sledding, because the Russians had... six months to dig in. And so those defensive belts that the Russians have put in place in the east and in the south are hard -- they'd be hard for any military to punch through."

"We want to make sure that the Ukrainians have sufficient artillery to keep them in the fight in the context of the current counteroffensive, and because things are going a little slower than some had hoped, there are very high expenditures of artillery."

But rights groups have come out strongly against the United States providing the munitions.

Human Rights Watch said that "transferring these weapons would inevitably cause long-term suffering for civilians and undermine the international opprobrium of their use opposes."

And Amnesty International said Biden's administration "must understand that any decision enabling the broader use of cluster bombs in this war will likely lead to one predictable outcome: the further death of civilians."

"Cluster munitions are an indiscriminate weapon that presents a grave threat to civilian lives, even long after a conflict has ended. Their transfer and use by any country under any circumstances is incompatible with international law," it added.

UN Secretary-General Antonio Guterres also opposed the move, with a spokesperson saying he "does not want there to be continued use of cluster munitions on the battlefield."

Russian officials issued no immediate response.

As the war passed the 500-day mark, the United Nations condemned the civilian cost inflicted.

More than 9,000 civilians, including over 500 children, have been killed since Russia's February 24, 2022 invasion, the UN's Human Rights Monitoring Mission in Ukraine said in a Friday statement, though rights experts have previously warned the real count is likely far higher than what is being tallied.

The monitoring mission said three times as many civilians were killed in the last 500 days as during the previous eight years of hostilities in eastern Ukraine. 

Kremlin watching closely

Zelensky's talks in Turkey -- a strategic member of NATO on uneasy terms with the West -- were being watched closely by the Kremlin, which has tried to break its international isolation by cultivating strong relations with President Recep Tayyip Erdogan.

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 negotiations, Erdogan risked drawing the ire of Russian President Vladimir Putin by delivering unequivocal support for Ukraine's NATO aspiration.

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

While Zelensky is pressing for NATO membership "now", the White House has urged restraint and said plainly it would not happen at next week's summit in the Lithuanian capital Vilnius.

Erdogan meanwhile said he will personally brief Putin on the negotiations when the Kremlin chief makes his first visit to Turkey since the invasion next month.

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

'Progress' on nuclear inspections

The head of the UN's nuclear watchdog said on Friday that it was "making progress" on inspecting several areas of the Zaporizhzhia nuclear plant in Ukraine, after claims it had been mined.

Ukraine and Russia have accused each other of planning a provocation at the Russia-controlled site, raising alarm over the threat of radioactive disaster at Europe's largest nuclear plant.

Ukraine's military this week claimed "external objects similar to explosive devices" had been placed on the outer roof of the third and fourth reactors at the site.

Officials from the International Atomic Energy Agency had been able to "complete the tours of the cooling ponds and other places", IAEA chief Rafael Grossi said in Tokyo.

They had "not seen any indications of explosives or mines", he said, although he added IAEA officials had not yet been able to visit the facility's rooftops.

Rescuers on Friday found a 10th body in the rubble of buildings in Lviv after the biggest Russian missile attack on civilian infrastructure in the western Ukrainian city since the invasion, its mayor said.

The strike also wounded 42 people, including three children, Ukraine's interior ministry said.