Speaking as an EU-Ukraine summit came to a close in Brussels on Monday, Tusk said that a rift between the countries would only serve Russia.
Ties between Poland and Ukraine have become strained over their shared history.
The countries held separate commemorations of the anniversaries of WWII tragedies last weekend.
Polish President Andrzej Duda took part in observances in Ukraine on Sunday marking the 75th anniversary of the Volhynia Massacre.
According to Poland’s Institute of National Remembrance (IPN), which is charged with prosecuting crimes against the Polish nation, the massacre saw some 100,000 Poles killed between March 1943 and the end of 1944 by the Ukrainian Insurgent Army (UPA) in Nazi German-occupied Poland.
Also on Sunday, Ukrainian President Petro Poroshenko visited the eastern Polish village of Sahryń, where hundreds of Ukrainians were killed by soldiers of Poland's underground Home Army in 1944, public broadcaster Polish Radio's IAR news agency reported.
EU-Ukraine summit
In summing up the European Union-Ukraine summit, which finalised the ratification of the EU-Ukraine Association Agreement, Tusk addressed the Kiev, saying: “I can see clearly that you will not be defeated by an external enemy. You are too strong. You can only be defeated by yourselves”.
He urged Ukraine to “keep your unity at any cost, and avoid like the plague internal conflicts”.
He also addressed Europe, saying that “only united can we overcome the challenges of modern times” and urged Poland and Ukraine to find a new approach in building relations.
Tusk also said the friendship between the European Union and Ukraine was “strong and deepening”.
He noted that during the summit the European Union finalised the ratification of the EU-Ukraine Association Agreement, which will boost Ukraine's political and economic integration with the bloc.
Tusk noted that the trade part of the deal, implemented provisionally, had already seen trade grow by 25 percent, and that Ukrainians have been allowed to travel to the European Union visa-free since a month ago.
The European Council chief also condemned Russian aggression and the annexation of Crimea, and called for the “immediate release of all illegally detained Ukrainian citizens in Russia and on the Crimean peninsula”. (vb/pk)