Russia has been criticized by Western leaders for “destabilizing” the impact on the crisis in Ukraine at the start of the NATO summit in Wales.
NATO and Britain have warned that pressure on Russia will increase if it does not change course in eastern Ukraine.
Ahead of the summit, Ukrainian President Petro Poroshenko briefed US and EU leaders on preliminary discussions on a ceasefire plan with Russian President Vladimir Putin.
About 2,600 people were killed in fighting between Ukrainian troops and insurgents.
The West says it has evidence that Vladimir Putin supports the separatists with training and armaments, and has sent Russian troops across the border. Russia denies the accusation.
The UN estimates the conflict has displaced more than a million people from homes in eastern Ukraine.
Ukrainian government forces recently suffered several casualties after insurgents launched an offensive in both Luhansk and Donetsk oblasts, as well as south around the city of Mariupol on the Sea of Azov.
There are reports that the separatists began shelling the external defense of Mariupol.
At least two military vehicles were spotted in the area, and eyewitnesses spoke of the shooting.
As a separate event, President Petro Poroshenko said that the “implementation” of his peace plan, which includes a bilateral ceasefire, could begin on September 5.
Petro Poroshenko said it depended on talks in Minsk between representatives of Ukraine, Russia, pro-Russian insurgents and the Organization for Security and Co-operation in Europe (OSCE).
During the two-day talks, NATO leaders are also due to discuss the rise of the Islamic State (ISIL) and Afghanistan, where Taliban militants launched a deadly attack on the government complex on September 4.
Earlier today, NATO Secretary General Anders Fogh Rasmussen said that the summit was being held in conditions of “radically changed security conditions” and that Russia was “attacking Ukraine.”
“Unfortunately, we still see Russia’s involvement in destabilizing the situation in eastern Ukraine,” he told reporters in Newport ahead of the official start of the summit.
Correspondents believe the summit has been the most important for NATO for decades, as leaders have faced the question of whether the alliance is ready to address the challenges of the 21st century.
NATO is expected to approve plans to build a rapid reaction force consisting of several thousand troops from member states capable of deploying within 48 hours.
European leaders are also going to discuss a new round of tougher economic measures against Russia.