Ukraine is facing "brutality" unseen in Europe "since World War II" amid Russia's invasion, said Jens Stoltenberg on the opening day of a pivotal NATO summit in Madrid.
The comments come a day after a Russian attack on a crowded shopping centre at Kremenchuk in central Ukraine killed at least 18 people, according to a provisional toll. The strike was described as a "war crime" by G7 leaders.
NATO countries, which have already supplied billions of euros worth of weapons to Kyiv, will agree in Madrid "a comprehensive programme of assistance" to help Ukraine "enforce its right to self-defence", Stoltenberg said at a briefing alongside Spanish Prime Minister Pedro Sánchez.
"It is extremely important that we are ready to continue to provide support because Ukraine is now facing a brutality that we have not seen in Europe since the Second World War," said Stoltenberg, NATO's secretary-general.
Sánchez said the summit would be an opportunity to send "a message of unity... to defend democracy and its values", defending "an international order based on rules".
The military alliance summit, which runs until Thursday, brings together more than 40 heads of state and government in the Spanish capital. It will be largely devoted to the war Russia launched against Ukraine on 24 February.
On Monday, Stoltenberg announced a massive increase in NATO's rapid reaction troops from 40,000 to 300,000, in a major reinforcement of the alliance's eastern flank.
Leaders of the Group of Seven major economies are travelling to Madrid from Bavaria in Germany, where they wrapped up a G7 summit pledging continued support for Ukraine "for as long as necessary".
Their final statement underlined their intent to impose “severe and immediate economic costs” on Russia. But it left out key details on how a proposal for price caps on Russian fossil fuels would work in practice.
Emmanuel Macron called on Tuesday for NATO to send a "message of unity and strength" at its summit, which is also due to decide whether Finland and Sweden will join the alliance.
"The message that must come out of Madrid is one of unity and strength for the countries that are members as well as for those who aspire to join it and whose approach we support," the French president told a news conference at the close of the G7 summit in Bavaria.
"We must be united in Madrid, united in our support for Ukraine, united in our firmness, united in the defence and security of the eastern flank of our alliance," he said. "It is the security of the European continent that is at stake," he stressed.
Turkey is blocking the accession of Sweden and Finland, which are accused of harbouring militants of the Kurdish PKK. Ankara has a list of grievances and wants guarantees as well as the extradition of more than 30 people by Finnish and Swedish authorities.
Jens Stoltenberg, who is due to take part on Tuesday in Madrid in a meeting on Tuesday between Turkish President Erdogan, his Finnish counterpart and the Swedish Prime Minister, said he hoped "to make progress" on the issue.
Erdogan also announced on Tuesday a possible meeting with US President Joe Biden "tonight or tomorrow" in Madrid, just hours before the summit.