Ukraine signed a deal with the EU in Brussels on Friday, committing both to closer political ties, as President Putin signed legislation in Moscow to incorporate Crimea into Russia.
Ukrainian Prime Minister Arseny Yatseniuk and EU leaders signed the core chapters of the Association Agreement during the EU spring summit.
More substantial chapters regarding trade will be signed by Kiev and Brussels after presidential elections in Ukraine scheduled for 25 May.
Poland's president Bronislaw Komorowski, currently on a trip to Budapest, said that the signing of the agreement commits Ukraine to implement a whole series of reforms.
"Therefore, in my opinion, the signing of this agreement must be read as an incentive for Ukraine to continue its efforts of rapprochement with the West," he said.
Meanwhile, the United States is preparing military exercises in Poland that could involve Czech Republic, Hungary, Slovakia, Romania, Bulgaria and the Baltic states, the US ambassador to Poland said on Friday.
US ambassador in Warsaw Stephen Mull told Radio Zet that "the US is preparing
large military exercises in the Lask [air] base," where 12 US F-16s and support staff arrived in March.
As the EU and Ukraine pledged closer ties, President Putin Putin ratified a treaty making Crimea part of Russia, as well as legislation creating two new Russian administrative districts: Crimea and the port city of Sevastopol.
"I want to congratulate all citizens of the Russian Federation - residents of the whole country, Crimea and Sevastopol - on what is, without any exaggeration, a banner event," Reuters reports the Russian president as saying.