The EU said Thursday that major new clashes in Ukraine were the biggest breach of a truce signed in February and could lead to a new spiral of violence between government forces and pro-Russian rebels.

"The violent fighting in Maryinka near Donetsk in the east of Ukraine constitutes the most serious violation of the ceasefire arising from the Minsk accords in February," European Commission spokeswoman Maja Kocijancic told reporters.

She added that it followed the movement of a "large quantity of heavy weapons towards the line of contact" by separatist rebels.