Western leaders have criticised Russia for its "destabilising" influence on the crisis in eastern Ukraine, as they opened a Nato summit in Wales. They said pressure on Russia would be increased if it did not change course. Russia denies claims that it arms rebels and has troops in Ukraine. Separately, Ukrainian President Petro Poroshenko and rebel leaders said a ceasefire could be agreed on Friday. Some 2,600 people have died in fighting between Ukrainian troops and rebels. Ukraine's military said 837 its soldiers have died since the conflict erupted in April.