NAIROBI (Reuters) - The United States said it was concerned about accounts of violence and intimidation in the run-up to Aug. 12 by-elections in Tanzania.

Tanzanian opposition leaders have complained that tolerance for dissent has diminished rapidly since President John Magufuli took office in 2015 on pledges to reform the East African nation’s economy and crack down on corruption.

The U.S. Embassy in Dar es Salaam cited “credible” accounts of violence and irregularities ahead of the by-elections.

In a statement published on its website on Wednesday, the embassy said the National Election Commission had refused to register opposition candidates and that they had been subject to police intimidation.

The embassy also cited unwarranted arrests of candidates and acts to suppress freedom of assembly and speech.

“Such actions undermine the rights that Tanzania’s constitution guarantees its citizens and jeopardize peace, stability, and security in the country and throughout the region,” the embassy said.

Magufuli’s government has introduced anti-corruption measures and tightened regulations on foreign companies, particularly in the mining sector.[L8N1M621N]

The vote in question involved a parliamentary by-election in the northwestern region of Kigoma and 36 local races, according to The Citizen, an English-language Tanzanian daily.

The paper reported that the electoral commission assessed the polls to be free and fair. The commission had asked the U.S. Embassy to provide evidence for its allegations, it said, citing NEC Information Officer Christina Njovu.

Electoral commission officials could not be reached for comment.