European Union officials and the Ukrainian government have signed a €1.3bn financial aid agreement to provide assistance to Kyiv.
Speaking at a press conference after the agreement, European commission president José Manuel Barroso said, "It is... critically important that we stop the spiral of political and security escalation which we have seen over recent weeks."
"We need to build on the diplomatic efforts agreed in the Geneva statement and continued by the OSCE. The so-called referendum held in Ukraine's eastern regions is illegitimate and illegal."
"We will not recognise its outcome. The rights of national minorities and all persons need to be respected and uphold, but in the framework of a united and unitarian Ukrainian state," he continued.
The Portuguese official met with Ukraine prime minister Arseniy Yatsenyuk in Brussels on Tuesday to secure an assistance deal for Ukraine to help pay its energy bills, combat corruption and reform its institutions.
"Ukraine can count on the European Union now and in the future" - José Manuel Barroso
The Portuguese official assured Kyiv that, "Ukraine can count on the European Union now and in the future".
Yatsenyuk thanked the EU, saying "Russia will fail to make Ukraine a failed state."
Barroso called on Russia to work toward stability in Ukraine as promised at the Geneva conference in April, adding that Moscow must "repeal the mandate of the federation council to use force on Ukrainian soil and act as a responsible stakeholder of our community of nations".
On Monday, the EU expanded its sanctions against the country, threatening more and tougher economic measures in the event of Russian interference in the Ukrainian presidential election on 25 May.
Both EU and Ukrainian leaders agreed that the presidential election must take place as planned, with Yatsenyuk saying this would be possible in most regions of the country apart from the city of Slovyansk, which is currently under the control of rebels, and some areas of eastern Ukraine.
"We still maintain stability in southern Ukraine, despite the fact that Russia tried desperately to trigger [actions there]," said the prime minister.
However, he acknowledged that "there will be some hot spots where it will be difficult to hold free and fair elections."
Barroso called on Russian president Vladimir Putin to engage in trilateral talks with Kyiv and Brussels on gas prices and the security of the Russian energy supply, saying, "It's not good for the future of Russia to have this kind of confrontation with the European Union. The European Union is by far the most important economic partner of Russia, in trade and investment."