By Ivana Sekularac

BELGRADE, April 11 (Reuters) - Serbia's electricity monopoly is considering building new power plants jointly with Russia's Inter RAO, sources told Reuters on Friday, in a deal that would give the Russians partial control of Serbia's electricity market.

A source close to the negotiations said the protocol will be signed on Monday by Elektroprivreda Srbije (EPS) and Inter RAO, which is jointly owned by Russia's former monopoly Unified Energy System (UES) EESR.MM and state nuclear energy corporation Rosatom.

"It is not a binding document," the source said, on condition of anonymity. "The protocol says the parties will do joint projects but there is no precise information on the projects nor on the investment share."

If a tie-in goes through, it will be the second major energy deal this year between Serbia and Russia, the Balkan country's main big-power ally in the issue of Kosovo, a former breakaway province which seceded two months ago with Western backing.

In January, Belgrade and Moscow signed a deal that saw Serbia included in the South Stream gas pipeline in return for Russia's Gazprom's (GAZP.MM: Quote, Profile, Research) taking a stake in Serbian oil monopoly NIS, a pact many analysts saw as politically motivated.

A senior Serbian government source confirmed a deal was in the works, but noted that government approval could only come from the cabinet that takes over after a general election next month, not the current caretaker government.

Polls show the May 11 election will be a neck-and-neck race between pro-Western liberals and nationalists who favour turning the country towards Russia.