U.S. President Donald Trump's national security adviser John Bolton has said the United States is ready to help Ukraine counter Russian "meddling" during its 2019 presidential election.

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"President Poroshenko and I agreed that we would look at steps that the United States and Ukraine could take to look at election meddling here and we can do that through normal law enforcement channels and through other mechanisms," Bolton said at a briefing in Kyiv following his meeting with Ukrainian President Petro Poroshenko.

"It's a threat that a number of countries in the West have seen, we've made it clear what our position is. And so we are prepared to work with the government of Ukraine to prevent such meddling here," he said.