The United States urged Armenia on Tuesday to investigate any credible reports of irregularities in a weekend referendum in which preliminary results showed voters approving a strengthening of the prime minister's powers.
The changes envisaged in Sunday's referendum, which are due to come into force after elections in May 2017, would curb the role of the historically powerful president and give more authority to the Armenian prime minister and parliament.
The opposition has said the changes are a ruse to let President Serzh Sarksyan take on an enhanced prime ministerial role at the head of the Republican Party after his presidential term ends in 2018. He has denied that. His supporters have said the changes are needed to prevent political instability.
According to preliminary results, 63.35 percent of people who voted Sunday supported the proposed changes, more than the simple majority needed.
Observers from the Council of Europe rights group reported problems with voting lists and other irregularities, and said the low turnout suggested many voters saw the referendum as a piece of political maneuvering.
"We are concerned by allegations of electoral irregularities reported by nonpartisan observers and various Armenian political parties," State Department spokesman John Kirby told reporters.
"We urge Armenian authorities to investigate fully all credible reports ... of irregularities to ensure the integrity of the referendum's outcome," he added, saying the issues raised were similar to those reported in previous Armenian elections and should be addressed to maintain public confidence in the vote.