Russia's invasion of Ukraine has sparked a "fundamental shift" in NATO's approach to defense, and member states will have to boost their military spending in an increasingly unstable world, the leader of the alliance said Tuesday.

Secretary-General Jens Stoltenberg spoke as U.S. President Joe Biden and other NATO leaders began to arrive in Madrid for a summit that will set the course of the alliance for the coming years. He 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.

Top of the agenda for leaders in meetings Wednesday and Thursday is strengthening defenses against Russia and supporting Ukraine.

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 also 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.

Diplomats and leaders from the three countries have held a flurry of talks in an attempt to break the impasse. The three countries' leaders are due to meet in Madrid alongside Stoltenberg on Tuesday.

Swedish Foreign Minister Ann Linde told the Svenska Dagbladet newspaper that negotiations with Turkey had "progressed" and that "something positive" might happen in Madrid, but "it can also take longer."

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.

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.

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 over Sweden and Finland 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.

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 to 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 alongside Russia.

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."