The number of African migrants entering Israel illegally through Egypt has dropped sharply since Egyptian political upheaval began in January, the Interior Ministry said on Tuesday.

About 700 migrants crossed through the largely unfenced border between Israel and Egypt's Sinai peninsula in the past two months, less than half the average monthly number in 2010, a ministry spokeswoman said.

"We don't know for certain why the sudden drop, but believe it is related to events in Egypt," she said.

Israeli human rights group attributed the decrease to violence in the Sinai between Egyptian police and Bedouin who smuggle the migrants across the border.

Alarmed by what it described as a near-doubling in the influx of Africans seeking work or claiming refugee status, Israel last year began erecting a fence along the frontier and announced plans to build an internment camp for border-jumpers.

Defense Minister Ehud Barak said on Tuesday that work on the fence was being speeded up to curb illegal migration and that Israel was on "high alert" along its Egyptian border.

Rights groups say many African migrants en route to Israel face torture, rape and assault by traffickers in the Sinai who hold them for weeks, sometimes months, to demand more money.

Israeli Physicians for Human Rights said in a survey of 284 migrants published last week that more than half told of abuse by the smugglers that included being burnt, branded, hung by the hands or feet and raped.

PHR's Executive Director Ran Cohen said the group was deeply concerned about some 920 people believed to be held in the Sinai and feared it would be unable to contact Egyptian authorities to locate and release them.

An Israeli court convicted an Eritrean migrant on Monday of conspiracy and extortion, saying he had acted as a middle-man for the Sinai smugglers.

The Tel Aviv court said Younis Zirisinei had played over the phone the cries and begging of a young Ethiopian woman, held and abused by the traffickers in Sinai, to her relative in Israel.

The woman told her relative to give the smugglers more money or they would kill her. Zirisinei, who will be sentenced at a later date, passed thousands of dollars from the man over to the traffickers, and the woman was released.