World leaders are due to meet Ukraine's president before the Nato summit gets under way in Wales.
President Petro Poroshenko will update US and EU leaders after discussions with Russia's President Vladimir Putin.
France has ramped up the pressure on Putin by axing its contract to deliver two navy assault ships to Russia.
The two-day Nato summit is expected to be dominated by the crisis in Ukraine, but leaders will also discuss the rise of Islamic State, and Afghanistan.
Poroshenko will brief UK PM David Cameron, US President Barack Obama and the leaders of France, Italy and Germany, in what correspondents say is a clear show of support to Ukraine's leader.
On Wednesday, President Poroshenko said he had agreed a "ceasefire process" during telephone talks with Putin, who said he hoped a peace deal could be reached when rebels meet envoys from both states on Friday.
But in an interview with the BBC, Nato chief Anders Fogh Rasmussen said the ceasefire plan floated by Putin was "insincere".
"I think the bottom line is that the Russians are not sincerely interested in a ceasefire. They continue to destabilize Eastern Ukraine," he said.
The BBC's Bridget Kendall says the summit is Nato's most important for decades, with its leaders faced to ask if the alliance is really equipped to deal with 21st century challenges.
The alliance is expected to approve plans to create a rapid response force composed of several thousand troops from member states, able to deploy within 48 hours.
European leaders are also set to discuss a new round of tougher economic sanctions on Russia.
French President Francois Hollande signalled a toughening of European policy towards Russia on Wednesday by cancelling a military deal worth 1.2bn euros ($1.6bn; £0.95bn).
Hollande's office blamed Russia's recent actions in Ukraine when it announced it was halting the delivery of the first of two Mistral navy assault ships.
President Obama has urged Nato members to send an unmistakable message of support to Ukraine in the face of what he called "brazen aggression" by Russia.
Speaking in Estonia while en route to the Wales summit, he said the vision of a peaceful and united Europe was threatened by Moscow's attempt to re-draw borders "at the barrel of a gun".
Shortly after Obama's comments, the Pentagon announced that the US was sending about 200 troops to Ukraine to take part in military exercises later this month in a show of solidarity.
Some 2,600 people have been killed in five months of fighting between Ukraine's army and rebels in the east of the country.
The West says it has evidence that President Putin is supporting the rebels with training and arms but Russia denies this.
Earlier this week, the UN said the conflict had forced more than a million people from their homes in eastern Ukraine.