NAIROBI (Reuters) - Rwanda has expelled about 400 Burundians back to their country accusing some of them of espionage and fuelling tensions between the two small neighbors whose relations have been strained by Burundi's political crisis, an official said on Sunday.

It's the second such expulsion in about a month and brings the total number of Burundians deported over the period to at least 1,700.

One of those expelled, a young man who did not wish to be named for fear he might be targeted, told Reuters Rwandan officials accused some of them of spying for Burundi.

"We were accused of being envoys of Burundi government and sent there to spy on Rwanda," he said.

Renée Mukandori, a Burundi local government official, confirmed the expulsion to Reuters and said it occurred on Thursday and Friday. Those deported mostly came from the Bugabira district of northern Burundi.

Burundi has accused Rwanda of interfering in its political crisis - which has seen Burundian government forces clash with protesters and rebels who say the president violated the constitution by standing for a third term last year.

Rwanda has denied Burundi's accusations.

But the violence in Burundi has rattled Rwanda and other countries across the central African region were there are still fresh memories of the 1994 genocide in Rwanda, which like Burundi has a Hutu majority and Tutsi minority.

Rwandan President Paul Kagame, a Tutsi, has said in the past he would not allow a repeat of the genocide in the region and has been critical of the handling of the crisis by President Pierre Nkurunziza, who led a Hutu rebel group in Burundi's civil war.

About a quarter of million people have fled the violence in Burundi and most of them are now in refugee camps in Rwanda and Tanzania.

Gille, another man who was sent back to Burundi, said those expelled were not newly arrived refugees and that they had come to Rwanda years ago seeking economic opportunities.

"We went to seek for better life. Once we got there, some of us rented fields and cultivated...we were not given time to go to harvest what is on our land," he said.

Rwandan officials could not be immediately reached but foreign minister, Louise Mushikiwabo, told reporters last week the Burundians being expelled were in the country illegally.

(writing by Elias Biryabarema; editing by Ros Russell)