Prior to the start of the summit, which was delayed as some of the leaders were late to arrive, the EU's foreign policy coordinator, Federica Mogherini, described the agreement reached in marathon talks in Minsk earlier on Thursday as important but not definitive.

"We are now working ... to trigger all means at the EU's disposal to facilitate the implementation of the agreements," Mogherini said. She added that she did not expect the leaders to discuss new sanctions on Russia at the summit, saying that instead of this, she was more interested in examining "positive ways in which the EU can contribute" to the implementation of the ceasefire in eastern Ukraine, which is to come into force on Sunday.

As they arrived in Brussels, the leaders expressed varying degrees of hope and skepticism about the deal, which was the culmination of a peace offensive launched by German Chancellor Angela Merkel and French President Francois Hollande last week.

'Glimmer of hope'

Before leaving Minsk to head to Brussels, Chancellor Merkel remained cautious, in light of the failure of a September agreement to end the bloodshed, describing Thursday's deal as no more than a "glimmer of hope."

"Hope is good, even essential, but hope is not enough," warned European Council President Donald Tusk, who is also the former prime minister of Poland. "The real test is the respect of the ceasefire on the ground," said Tusk, who held talks with Ukrainian President Petro Poroshenko (pictured above), shortly after he arrived in Brussels from Minsk.

British Prime Minister David Cameron said the onus was on Russian President Vladimir Putin to ensure that pro-Russia separatists complied with the terms of the deal.

"We should be very clear that Vladimir Putin needs to know that unless his behavior changes, the sanctions we have in place won't be altered," Cameron said, referring to measures taken against individuals accused of supporting the rebels.

Hollande described the next few hours as decisive.

"We will have to remain vigilant, to maintain the pressure and to press ahead," he said upon arriving in Brussels from Minsk.

Provisions of the agreement

The agreement reached in Minsk on Thursday calls on both the pro-Russia rebels and Ukrainian government troops to observe a ceasefire beginning at midnight Kyiv time on February 15. It also calls for the withdrawal of all heavy weapons from the frontline to create a buffer zone 50 kilometers (30 miles) wide - and even wider for certain types of rocket or missile-launching systems. It also calls for all foreign fighters and military equipment to leave Ukrainian territory as well as the release of all hostages. All this is to be monitored by teams from the Organization for Security and Cooperation in Europe.

Politically, the accord directs the two sides to open talks aimed at organizing local elections in the east, which would comply with Ukrainian law, with Kyiv also to take steps to grant parts of the east of the country more autonomy.

More than 5,300 people have been killed since the fighting broke out in Ukraine last March.

pfd/ (Reuters, AP, dpa, AFP)