BISHKEK, Kyrgyzstan (AP) — Voters in Kyrgyzstan are casting ballots in a constitutional referendum that includes amendments that boost the power of prime minister — something opposition groups have criticized in the ex-Soviet Central Asian nation.

Sunday's vote was initiated by President Almazbek Atambayev. His critics have claimed the move could allow him to shift into the prime minister's seat after his current term as president ends next year but Atambayev denies such intentions.

Political infighting ahead of the referendum has fueled fears of a new round of instability in the impoverished nation of six million on China's mountainous western frontier. Kyrgyzstan has seen the violent overthrow of two presidents since gaining independence in 1991.

If approved, the amendments would allow the prime minister to appoint and dismiss Cabinet ministers and regional governors without consulting the president. The prime minister would also have a decisive say in shaping the government budget.

Atambayev's supporters cast the amendments as part of efforts to increase the power of parliament and strengthen checks and balances between branches of government.

The amendments also envisage a ban on same-sex marriages.

Kyrgyzstan used to host a U.S. air base that supported military operations in Afghanistan, but it evicted the U.S. in 2014. It still hosts a Russian air base and is part of a Moscow-dominated economic and security alliance of ex-Soviet nations.