Nato Secretary General Anders Fogh Rasmussen has accused Russia of "attacking Ukraine" and called on it to "stop the flow of weapons and fighters" into the country.
"We are still witnessing ... Russian involvement in destabilising the situation in eastern Ukraine," he said in remarks opening the summit.
He added that Ukraine's President Poroshenko and Nato will be issuing a joint statement, and confirmed they are looking at ways to "step up cooperation".
Angus Raffle, 8, says his surprise meeting with the President at Stonehenge was "awesome". His mother says she "squealed like a teenager".
The language has been ramped up at this Nato summit, yet caution appears the order of the day after the mistakes of recent history.
The US President's motorcade pulled up at the school with his black stretch limousine bearing a Welsh flag for the first time in history.