Here are the key takeaways from the talks in Washington. Given his acrimonious last visit to the Oval Office in February, the Ukrainian president went to considerable lengths to be collegial and charm his American hosts - including a flurry of six "thank yous" within the first few minutes of the meeting.

The fast-moving developments in Trump's near-back-to-back summits with the heads of Russia, Ukraine and European powers have highlighted a pair of repeated sticking points.