"We have watched over the last several months as the people of Ukraine have declared their desire for a sovereign, independent economic and foreign policy that promises them freedom and prosperity and self-determination," Obama said at a joint media appearance with his Ukrainian counterpart Petro Poroshenko on Wednesday.

"Unfortunately what we've also seen is Russian aggression, first in Crimea, and most recently in portions of Eastern Ukraine that have not only violated the sovereignty and territorial integrity of the country, but have also been designed to undermine these critical reform efforts that have been taking place in Ukraine," Obama said.

United States, he said, has led the international community and the European partners in making sure that there is a cost to Russia for its aggression in Ukraine.

"We've applied some of the toughest coordinated sanctions that we've ever seen between the Trans-Atlantic community, which are having a significant impact on the Russian economy," said US President.

Obama said he reaffirmed his commitment to Ukraine and US is providing additional assistance, both economic and security to Ukraine.

"We are going to continue to seek to mobilise the international community to say to Russia that Ukraine desires to have a good relationship with all of its neighbours, both East and West. And that there should be a way in which Ukraine is able to negotiate and then trade and continue their people-to-people links between Ukraine and Russia, but that Russia cannot dictate to them their ability to work effectively with other partners in order to better the situation for the Ukrainian people," he said.

Poroshenko said he is impressed by the level of bipartisan support which was demonstrated in the Congress.

"I want to thank you to the President for his leadership in the world, for protecting Ukraine's sovereignty, territorial integrity and independence. This is a true leadership defending democracy and freedom," he said.

The two leaders, he said, discussed a wide range of issues, including energy.

Earlier in the day, White House Press Secretary Josh Earnest said US security assistance being provided to Ukraine include the kind of valuable equipment that will be useful to the Ukrainian military.

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