JERUSALEM, Dec 13 (Reuters) - Israel has been quietly repatriating scores of Sudanese migrants with the help of a non-governmental organisation that routes them through a third country, an official involved in the transfer said on Monday.

Relatively few migrants have been returned, but there could be a broader campaign by the Jewish state, worried about the demographic impact of the estimated 35,000 Africans who have entered illegally through Egypt in the last few years.

Some 200 Sudanese who agreed to go home have been given cash stipends and flown out in recent months, and a further 150 will follow this week, the official said on condition of anonymity.

As Sudan is technically at war with Israel, the migrants go to another country from which they travel separately to their native land, the official said. Their travel is handled by a foreign NGO and coordinated with the United Nations High Commissioner for Refugees (UNHCR).

The official gave no further details of the procedure, citing concern for the migrants' safety back in Sudan.

An Israeli foreign ministry spokesman declined comment.

We are aware that a small number of people have returned to Sudan, said the UNHCR representative in Israel, William Tall. We can also say there was no coercion involved.

Israel, population 7.2 million, says the migrants from Sudan, Eritrea and other African countries come mostly in search of work, rather than asylum. The UNHCR disagrees with the Israeli criteria for qualification as a refugee. To stem the influx, Israel has been erecting a fence along its porous desert border with Egypt and plans to set up a holding facility for migrants who are intercepted crossing it. (Writing by Dan Williams; editing by Tim Pearce)