Ukrainian President Volodymyr Zelensky signed a security pact with the European Union providing long-term assurances, at a summit of EU leaders in Brussels on Thursday.
Zelensky signed the accord in a ceremony along with European Council President Charles Michel and European Commission President Ursula von der Leyen.
Thanking EU leaders for their help, Zelensky called for more arms and stronger sanctions on the Kremlin as the "existing pressure on Russia" was not enough to stop the war.
The agreement between the EU and Ukraine was drawn up after months of talks among the 27 EU member states to hammer out the details. It is seen as a sign of unwavering EU support, despite political changes across the bloc that played out in this month's European elections.
As well as new plans for closer cooperation between the defence industry in Ukraine and the EU, a new EU-Ukraine crisis consulting procedure forms part of the pact.
Should Russia attack Ukraine again after the current invasion, or use nuclear weapons during the ongoing war, the EU and Ukraine are to hold emergency consultations within 24 hours at the request of either party to discuss possible support.
Existing EU initiatives like training Ukrainian soldiers are set out again, as well as long-term commitments to supply Ukraine with artillery and air defence systems to help Kiev against the full-scale Russian invasion.
Plans for greater cooperation defending against Russian cyber attacks and disinformation are also included.
Zelensky said the security agreement was an important step towards Ukraine's "historic goal of peace and prosperity" within the EU, in a message posted on X ahead of his arrival.
German Chancellor Olaf Scholz described the EU agreement with Ukraine as a "symbol of solidarity in difficult times."
It follows the official start on Tuesday of negotiations for Ukraine to eventually join the bloc.
EU membership is a powerful motivator for Ukraine in the country's fight to defeat the Russian invasion. Kiev also wants to join NATO in the future.
After the signing ceremony, Michel praised the Ukrainian president's leadership and said the EU would continue to stand by Ukraine for as long as necessary.
"We know that you are fighting for your freedom, you are fighting for your land, for your future, for your children, but you are also fighting for our common values and principles," he said.
The accord struck with the EU is similar to agreements going back to a NATO decision last July that individual allies would conclude bilateral deals to ensure Ukraine's long-term security.
Ukraine has already signed such arrangements with EU countries including Germany and France.
The United States and Ukraine signed their long-term security agreement at a recent G7 summit in Italy.
The agreements ensure that Ukraine will receive arms supplies and financial aid to help bridge the gap until the country's intended accession to NATO.
The timing of Zelensky's visit to Brussels was kept secret ahead of his arrival. For security reasons, Zelensky's travel plans are rarely announced in advance.
