Donald Tusk made the statement via social media on Tuesday.
On Monday, the Polish prime minister visited Kyiv for talks with Ukraine's President Volodymyr Zelensky and Prime Minister Denys Shmyhal.
In a Facebook post, Tusk said that Kyiv was the first European capital he had travelled to on a bilateral visit, because "Ukraine is fighting for the security of the entire world."
Poland's prime minister said he had told Zelensky he would "continue to take all steps" to increase Ukraine's chances of winning the war against Russia, "including in the European Union."
Tusk said he had declared Poland's support in Ukraine's accesion to the EU, so that Kyiv "becomes a full member" of the bloc "as soon as possible."
The Polish prime minister said he welcomed Zelensky's declaration that Poland and Ukraine would "carry out joint projects" in the "production of weapons and ammunition."
Warsaw and Kyiv also "reached mutual understanding" on the border situation and Ukrainian leaders reaffirmed their willingness to "seek shared solutions through bilateral talks" with the Polish government, Tusk said.
The Polish prime minister also declared Poland would aim to "make an active contribution to the reconstruction of Ukraine, including with the participation of Polish businesses," and told Ukrainian leaders he had appointed the MP Paweł Kowal as the Cabinet's commissioner for the rebuilding of Ukraine.
Meanwhile, the Ukrainian head of govrnment said Kyiv was open to granting concessions to Polish companies to take part in the country's key infrastructural projects, such as road-building and development of energy facilities, according to officials.
Tusk said he had invited Shmyhal and his ministers for "full-scale inter-governmental consultations" in Poland, to "continue these good talks."
In conclusion, the Polish prime minister reaffirmed Warsaw's support for Kyiv's defence against the Russian invasion and vowed Ukraine "won't be left alone," Polish Radio's IAR news agency reported.
Tuesday is day 699 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, Facebook/DonaldTusk