Authorities in Ukraine say security services will investigate the shelling of a school in the rebel-held east that left two teenagers dead and four others injured.

Security spokesman Andriy Lysenko said Thursday that preliminary information indicates the shells flew into the schoolyard in Donetsk from a location controlled by separatist forces.

But the rebels swiftly blamed Ukrainian forces for Wednesday's deadly attack, saying government troops indiscriminately target residential areas in eastern Ukraine.

The UN estimates more than 4,000 people have been killed over six months of fighting between Ukrainian government forces and pro-Russian separatists. Several hundred have died since a much-violated cease-fire was signed in early September.