Brussels is "ready to review" new sanctions it plans to impose on Russia next week if a truce between pro-Moscow rebels and Kiev holds, European Council President Herman Van Rompuy said Sunday.

"We have noted that Russia only consented with difficulty to serious negotiations. The ceasefire is an import step, but it is only a step," Van Rompuy told Belgian television channel VRT.

"We are ready if the ceasefire is durable, and/or if the peace talks start, to review these sanctions."

The new round of sanctions, agreed Friday, would tighten existing measures imposed in July, targeting more individuals with travel bans and asset freezes, as well as tightening access to capital markets for Russian oil and defence companies.

They will be formally approved on Monday, although the full details of the people and organisations affected will not be released for another day.

EU leaders, along with US President Barack Obama, agreed to impose the sanctions despite a truce signed on Friday between pro-Moscow rebels and Kiev to end fighting in eastern Ukraine.

On the ground, the agreement was already looking shaky on Sunday after fighting broke out around two flashpoint cities overnight, killing one woman and sparking renewed fear among residents.

"The most important thing was to achieve a ceasefire. There have been a few incidents. We have to wait a bit to see if these incidents will continue or if they are exceptions," said Van Rompuy.

"There is a ceasefire after nearly 2,000 have died, do not forget. Between that and peace, there is still a big step and negotiations should begin as soon possible," he added.

According to AFP calculations, the five-month conflict in Ukraine has claimed 2,800 lives and sent at least half a million fleeing their homes.

Russia said on Saturday that it "react" if the EU imposes new sanctions.

"Russia's economy is suffering. They are already in recession, mainly because of the sanctions," said Van Rompuy.