"The participation of yet another participant, even more so with the ambitions of the United States, will only further complicate the negotiations process and decision-making will be dragged on even longer," Pushilin said.
In an earlier interview with Russian radio station Ekho Moskvy, US Ambassador to Russia John Tefft said that Obama would join the Normandy format with pleasure.
The Normandy format consists of the heads of Russia, Ukraine, France, and Germany.
Kiev forces and Donbas militias signed the agreement, which envisioned the establishment of ceasefire, withdrawal of artillery from the line of contact, all-for-all prisoner exchange and a higher level of independence for Donetsk and Luhansk.
The Organization for Security and Co-operation in Europe (OSCE) is tasked with monitoring the ceasefire in Ukraine's southeast. On April 14, after Normandy format talks, German Foreign Minister Frank-Walter Steinmeier said all negotiations participants agreed to continue financial support for the OSCE monitors to take all necessary steps to stop the ongoing clashes in the region.