Heads of State and Government pose for the official group photo during the NATO summit in Madrid on Wednesday. AFP

At a summit dominated by the invasion and the geopolitical upheaval it has caused, NATO also invited Sweden and Finland to join and pledged a seven-fold increase from 2023 in combat forces on high alert along its eastern flank against any future Russian attack.

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In reaction, President Vladimir Putin said Russia would respond in kind if NATO set up infrastructure in Finland and Sweden after they join the US-led military alliance.

Putin was quoted by Russian news agencies as saying he could not rule out that tensions would emerge in Moscow's relations with Helsinki and Stockholm over their joining NATO.

Volodymyr Zelensky delivers a statement at the start of the first plenary session of the NATO summit in Madrid. AFP

US President Joe Biden announced more land, sea and air force deployments across Europe from Spain in the west to Romania and Poland bordering Ukraine.

These included a permanent army headquarters with accompanying battalion in Poland — the first full-time US deployment on NATO's eastern fringes.

"President Putin's war against Ukraine has shattered peace in Europe and has created the biggest security crisis in Europe since the Second World War," NATO Secretary-General Jens Stoltenberg told a news conference.

"NATO has responded with strength and unity," he said.

'Fighting everywhere'

As the 30 national NATO leaders were meeting in Madrid, Russian forces intensified attacks in Ukraine, including missile strikes and shelling on the southern Mykolaiv region close to front lines and the Black Sea.

The mayor of Mykolaiv city said a Russian missile had killed at least five people in a residential building there, while Moscow said its forces had hit what it called a training base for foreign mercenaries in the region.

Leaders pose for a picture during a NATO summit in Madrid, Spain, on Wednesday. Reuters

The governor of eastern Luhansk province reported "fighting everywhere" in a battle around the hilltop city of Lysychansk, which Russian forces are trying to encircle as they gradually advance in a campaign to conquer all of Ukraine's industrialised eastern Donbas region on behalf of separatist proxies. Donbas comprises Donetsk and Luhansk provinces.

Regional Donetsk Governor Pavlo Kyrylenko told Ukrainian television that Russian attacks killed one civilian and wounded eight on Wednesday.

‘Full solidarity’

In a nod to the precipitous deterioration in relations with Russia since the invasion, a NATO communique called Russia the "most significant and direct threat to the allies' security", having previously classified it as a "strategic partner".

NATO issued a new Strategic Concept document, its first since 2010, that said a "strong independent Ukraine is vital for the stability of the Euro-Atlantic area".

To that end, NATO agreed a long-term financial and military aid package to modernise Ukraine's largely Soviet-era military.

NATO Secretary General Jens Stoltenberg attends a news conference during a NATO summit in Madrid. Reuters

"We stand in full solidarity with the government and the people of Ukraine in the heroic defence of their country," the communique said.

Stoltenberg said NATO had agreed to put 300,000 troops on high readiness from 2023, up from 40,000 now, under a new force model to protect an area stretching from the Baltic to the Black seas.

Zelenskiy, in a video link-up with the summit, said Ukraine needed $5 billion per month for its defence and protection.

"This is not a war being waged by Russia against only Ukraine. This is a war for the right to dictate conditions in Europe — for what the future world order will be like," he said.

Nato's invitation to Sweden and Finland to join the alliance marks one of the most momentous shifts in European security in decades as Helsinki and Stockholm drop a tradition of neutrality in response to Russia's invasion.

Russia's stepped-up missile attacks in Ukraine and its forces were making slow but relentless progress.

In Mykolaiv, Mayor Oleksandr Senkevych said eight Russian missiles had struck the city, including an apartment block. Photographs showed smoke billowing from a four-storey building with its upper floor partly destroyed.

Russia's defence ministry said its forces carried out strikes on a military training base for "foreign mercenaries" near Mykolaiv and also hit ammunition fuel storage. Reuters was not able to independently verify the reports.

A river port and ship-building centre just off the Black Sea, Mykolaiv has been a bastion against Russian efforts to push westward towards Ukraine's main port of Odesa.

Oleksander Vilkul, governor of Kryvyi Rih in central Ukraine, said Russian shelling had increased there too.

Reuters