Secretary-General Jens Stoltenberg spoke as US President Joe Biden and other NATO leaders arrived in Madrid for a summit that will set the course of the alliance for the coming years.
The summit was kicking off with a leaders' dinner hosted by Spain's King Felipe VI at the 18th-century Royal Palace of Madrid.
Stoltenberg said the meeting would chart a blueprint for the alliance ``in a more dangerous and unpredictable world.''
``To be able to defend in a more dangerous world we have to invest more in our defense,'' Stoltenberg said. Just nine of NATO's 30 members meet the organization's target of spending 2% of gross domestic product on defense. Spain, which is hosting the summit, spends just half that.
Top of the agenda for leaders in meetings Wednesday and Thursday is strengthening defenses against Russia and supporting Ukraine.
US President Joe Biden, who arrived with the aim of stiffening the resolve of any wavering allies, said NATO was ``as united and galvanized as I think we have ever been.''
Moscow's invasion on Feb. 24 shattered European security and brought shelling of cities and bloody ground battles back to the continent. NATO, which had begun to turn its focus to terrorism and other non-state threats, has had to confront an adversarial Russia once again.
``Ukraine now faces a brutality which we haven't seen in Europe since the Second World War,'' Stoltenberg said.
Russia's invasion has prompted Sweden and Finland to abandon their long-held nonaligned status and apply to join NATO.
But they are being blocked by Turkish President Recep Tayyip Erdogan, who has insisted that he will only allow the Nordic pair to enter if they change their stance on Kurdish rebel groups that Turkey considers terrorists.
Stoltenberg said ``we hope to make progress'' on the issue in Madrid, but that was far from certain.
Diplomats and leaders from the three countries have held a flurry of talks in an attempt to break the impasse. The three countries' leaders met for more than two hours alongside Stoltenberg on Tuesday.
The Turkish leader showed no sign of backing down as he left Ankara for Madrid.
``We don't want empty words. We want results,'' Erdogan said.
Erdogan is critical of what he considers the lax approach of Sweden and Finland toward groups that Ankara deems national security threats, including the Kurdistan Workers' Party, or PKK, and its Syrian extension.
American support for Syrian Kurdish fighters in combatting the Islamic State group has also enraged Turkey for years.
Turkey has demanded that Finland and Sweden extradite wanted individuals and lift arms restrictions imposed after Turkey's 2019 military incursion into northeast Syria.
Sinan Ulgen, director of the Center for Economics and Foreign Policy Studies in Istanbul, was pessimistic about a breakthrough in Madrid.
He said the issue of the Kurdish groups was the main block, ``but there are other demands, and the involvement of other actors like the US could facilitate a deal.''
Biden is due to meet Erdogan Wednesday on the sidelines of the NATO summit. U.S. national security adviser Jake Sullivan said the Nordic nations dispute would not be the ``central focus'' of the meeting and the U.S. was not going to ``take on a brokering role.''
Ankara has been upset with the US since the Americans kicked Turkey out of its F-35 stealth jet program after Turkey bought Russian-made S-400 missiles. Turkey is waiting to hear back on a request for upgraded F-16s from the US.
Jamie Shea, a former senior NATO official who is an associate at the Chatham House think tank, said the Madrid meeting, with national leaders present in the media glare, ``is the moment of maximum pressure'' for compromise.
``It's either at Madrid or it's likely to go on for a long while,'' he said.
Ending the deadlock would allow NATO leaders to focus on their key issue: an increasingly unpredictable and aggressive Russia.
A Russian missile strike Monday on a shopping mall in the central Ukrainian city of Kremenchuk was a grim reminder of the war's horrors. Some saw the timing, as Group of Seven leaders met in Germany and just ahead of NATO, as a message from Moscow.
Ukrainian President Volodymyr Zelenskyy, who is due to address NATO leaders by video on Wednesday, called the strike on the mall a ``terrorist'' act.
Kyiv Mayor Vitali Klitschko traveled to Madrid to urge the alliance to provide his country with ``whatever it takes'' to stop the war.
``Wake up, guys. This is happening now. You are going to be next, this is going to be knocking on your door just in the blink of an eye,'' Klitschko told reporters at the summit venue.
Stoltenberg said Monday that NATO allies will agree at the summit to increase the strength of the alliance's rapid reaction force nearly eightfold, from 40,000 to 300,000 troops.
The troops will be based in their home nations, but dedicated to specific countries on NATO's eastern flank, where the alliance plans to build up stocks of equipment and ammunition.
Beneath the surface, there are tensions within NATO over how the war will end and what, if any, concessions Ukraine should make to end the fighting.
There are also differences on how hard a line to take on China in NATO's new Strategic Concept _ its once-a-decade set of priorities and goals. The last document, published in 2010, didn't mention China at all.
The new concept is expected to set out NATO's approach on issues from cybersecurity to climate change _ and the growing economic and military reach of China, and the rising importance and power of the Indo-Pacific region.
For the first time, the leaders of Japan, Australia, South Korea and New Zealand are attending the summit as guests.
Some European members are wary of the tough U.S. line on Beijing and don't want China cast as an opponent.
Stoltenberg said last week that ``we don't regard China as an adversary,'' but added that it ``poses some challenges to our values, to our interests, to our security.''
In the Strategic Concept, NATO is set to declare Russia its number one threat.
Russia's state space agency, Roscosmos marked the summit's opening by releasing satellite images and coordinates of the Madrid conference hall where it is being held, along with those of the White House, the Pentagon and the government headquarters in London, Paris and Berlin.
The agency said NATO was set to declare Russia an enemy at the summit, adding that it was publishing precise coordinates ``just in case.''