Russia's president said NATO's pledges at the Madrid summit showed it was not acting in Ukraine's interests or "the good of the Ukrainian people".
NATO's promise of continued support for Ukraine in the face of Russia's "cruelty" has brought an angry response from Vladimir Putin, who denounced the alliance's "imperialist ambitions".
At their summit in Madrid, members of the Western military alliance pledged to back Ukraine for as long as necessary, modernising the country's army to help it resist Russia’s invasion. NATO also updated its strategic plan to name Russia as the biggest "direct threat" to Western security.
The alliance has also approved membership applications from Finland and Sweden and announced a massive boost in troop numbers along its eastern flank.
But speaking during a visit to Turkmenistan, the Russian president blasted the West's attitude. "The leading countries of NATO want (...) to assert their hegemony, their imperial ambitions," he said.
"The call for Ukraine to continue fighting and refuse negotiations only confirms our assumption that Ukraine and the good of the Ukrainian people is not the goal of the West and NATO, but a means to defend their own interests," the Russian leader added.
NATO Secretary General Jens Stoltenberg said on Thursday that the "main message" from the summit was to make it clear that any aggression against "any inch of NATO territory" similar to Russia's actions against Ukraine "will trigger the full response from the whole alliance".
Warning that there was a "hot war" going on in Europe, he said NATO had to balance protection for Ukraine with a responsibility to avoid escalation.
"We also know that this can get worse. If this becomes a full-scale war between Russia and NATO, then we'll see suffering, damage, death, destruction at a scale which is much, much worse than what we see in Ukraine today," he added.
UK announces extra £1 billion for Ukraine
On the sidelines of the meeting in the Spanish capital, London and Washington stepped up their military and economic aid to Ukraine. The UK has announced an extra £1 billion (€1.16 billion) in military aid, bringing its total contribution to £2.3 billion (€2.67 billion).
The United States announced the payment of a tranche of $1.3 billion (€1.24 billion) in economic aid, as part of a $7.5 billion (€7.18 billion) support plan promised by Washington in May.
Ukraine's President Volodymyr Zelenskyy — who has branded Russia a "terrorist" state — hailed the outcome of the NATO summit, which he said would see the alliance "changing its strategy in response to Russia's aggressive anti-European policies".
In a joint statement, the NATO countries said they had agreed on a new aid plan involving the "delivery of non-lethal military equipment" and strengthening Ukraine's defences against cyber attacks.
"Russia’s appalling cruelty has caused immense human suffering and massive displacements," they wrote, saying Moscow bore "full responsibility for this humanitarian catastrophe".
Russia 'withdraws from Snake Island'
The Russian army said on Thursday that it had withdrawn from Snake Island, a strategic position in the Black Sea conquered by Moscow which had come under Ukrainian shelling in recent weeks.
The defence ministry in Moscow said the move was "a sign of goodwill" and aimed at facilitating grain exports.
But Ukraine's military said a barrage of Ukrainian artillery and missile strikes forced the Russians to flee the island in two speedboats.
Earlier on Thursday, authorities appointed by Moscow in Berdyansk — several hundred kilometres to the east in the Sea of Azov — said that a merchant ship carrying 7,000 tonnes of grain had left the Russian-occupied Ukrainian city, protected by the Russian navy and heading for "friendly countries".
Ukraine has accused Russia for weeks of stealing its wheat crops from areas occupied by the Russian army in southern Ukraine and illegally selling it internationally. Moscow denies any theft.
Ukrainian authorities said on Thursday they were trying to evacuate residents from Lysychansk, the focus of Russia's attacks in the east where about 15,000 people remain under relentless shelling.
"Fighting is going on all the time. The Russians are constantly on the offensive. There is no let-up," regional Governor Serhiy Haidai told Ukrainian television.
He later wrote on Telegram that Russian troops had attacked the Lysychansk oil refinery on Thursday morning. A senior separatist official told Russia's RIA news agency that the refinery was now fully controlled by Russian and pro-Russian forces, and all roads to the city were also under their control.
'All going to plan', says Putin
Speaking early on Thursday, Vladimir Putin said Russia's goals were the "liberation" of the Donbas region, the "protection" of its people, and the need to "guarantee the security of Russia itself".
He denied that Moscow's plans had had changed since the early stages of the war, when Russian forces failed to take Kyiv and withdrew from the region.
"Everything is going according to plan,” Putin said in Turkmenistan.
Funerals were due to be held on Thursday for some of the 18 people confirmed killed by Monday's Russian missile strike on a busy shopping centre in Kremenchuk. Crews have continued to search for another 20 people who remain missing.
A new report by Amnesty International concludes that a Russian air strike in March on a theatre in Mariupol — where civilians had taken shelter and hundreds are thought to have died — was a deliberate act. It says there was no evidence the building was used as a base for Ukrainian soldiers, as Moscow has claimed.