KIEV, Jan. 20 - Ukrainian President Petro Poroshenko called for a worldwide effort to counter the threat of Russian cyber warfare and urged the United States to "be great again" by demonstrating leadership on issues such as global security, Reuters reported.

U.S. President Donald Trump's pledge to improve ties with the Kremlin and open admiration for Russian President Vladimir Putin has put Ukraine, whose Crimea region was annexed by Russia in 2014, under the spotlight.

Poroshenko played down speculation that Washington could backtrack on its support for Kiev, noting that Trump had said publicly he would stick to U.S. obligations and there had been "promising" statements by nominees to his cabinet.

"That gives us a lot more optimism for the future," Poroshenko told Reuters on the sidelines of the World Economic Forum in Davos last week, adding he already had a visit planned to meet the new U.S. president "in a few months".

Poroshenko said joint global efforts were needed to halt Russian aggression, both military and cyber:

"There is a global cyber war of Russia against (the) whole world, there is lots of evidence. This is a global danger and the world should be together to fight this danger."

Ukraine's utility Ukrenergo told Reuters on Wednesday in Kiev that a cyber attack was behind a Dec 17-18 power blackout in the capital city.

Ukrenergo did not say who was behind the incident, although Ukrainian security services blamed Russian cyber attacks for similar power outages in December 2015. The Kremlin has denied any involvement in cyber attacks on Ukraine.

Moscow is also alleged to have sought to influence the U.S. election by hacking Democratic political groups, something Russia has dismissed.

"The same way as Russian propaganda is an element of Russian hybrid war, cyber (warfare) is an element of the Russian hybrid war, no matter if it's in Germany or United States," the Ukrainian president said.

Poroshenko also stressed the importance of NATO as a bulwark against Moscow after Trump stirred unease in Europe by calling the military alliance "obsolete.”

"NATO, mainly this is not a question of money, it is a question of security. Russian aggression demonstrated again there is no other security system but NATO which was effective to stop the aggression," he said.

And to demonstrate its role as a global leader, the U.S. would need to establish trans-Atlantic unity.

"America should be great again," he said in a reference to Trump's campaign catchphrase. (rt/ez)