Donald Tusk made the declaration at a news conference in the Ukrainian capital on Monday.
The Polish prime minister arrived in Kyiv in the morning and held talks with Ukraine's President Volodymyr Zelensky and Prime Minister Denys Shmyhal.
Poland's support for Ukraine's war effort against Russian invasion
Tusk told reporters afterwards that "Poland will do everything in our power to increase Ukraine's chances of winning this war."
The prime minister stressed that "there is nothing more important than supporting Ukraine in its war effort against the Russian attack."
He said the government in Warsaw would continue supporting Ukraine "for moral reasons," as well as "for the sake of Poland's national security."
Poland to help with Ukraine's EU bid
Poland's prime minister added he had agreed with Zelensky that Poland would "work to help Ukraine in all aspects of the process of its accession to the European Union."
Poland, Ukraine to produce arms together
Tusk also stated that "Poland and Ukraine are ready to finalise talks on investments in the joint production of ammunition and weapons."
The Polish prime minister said Warsaw's new government would appoint a special "commissioner for the reconstruction of Ukraine," and the role will likely go to the MP Paweł Kowal, Polish Radio's polskieradio24.pl website reported.
Earlier in the day, Tusk also laid a wreath at Kyiv's Wall of Remembrance of the Fallen for Ukraine.
Monday is day 698 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Source: IAR, polskieradio24.pl, Interia.pl