Ukrainian President Petro Poroshenko says that U.S. support has been critical for successful resistance to the Russian aggression.

"Ukraine highly appreciates a bond with the United States. The American leadership has been very important for progressive reforms in Ukraine. U.S. support has been critical for successful resistance to the Russian aggression," the president said at the 'Ukraine's Battle for Freedom Continues' forum in Washington on Thursday.

He called the United States to boost bilateral dialogue and upgrade the two countries' strategic alliance.

"It is high time to boost Ukraine-U.S. bilateral dialogue in form and content. It is also time to upgrade our strategic alliance and to channel our intense cooperation in defense and security into a comprehensive bilateral security agreement," he said.

"I believe that this would be to benefit of all," he added.

Poroshenko also called on the United States not to forget about values, namely freedom.

"In the times of global weakness, there is a thing that counts more than anything else – leadership. It is a privilege and a heavy burden. Not everyone wants it. Not everyone deserves it. There are so many reasons not to carry it: indifference, fatigue, self-doubt…And there is only one reason to carry it: you should have it in your blood. In these times of doubt, I urge America to be strong and believe in herself – just like people of goodwill worldwide keep believing in America. Don't let pragmatism make you forget about values, with freedom being at the top of them," he said.

"Don't let America become small-minded," he added.