EL-ARISH, Egypt (Reuters) - Egyptian border police shot dead two African migrants trying to slip into Israel, security sources said, pushing the number of African migrants killed this week at the sensitive border to four.

A 26-year-old Eritrean was shot dead on Thursday, while in a separate incident late on Wednesday a 27-year-old migrant of unknown nationality was killed, the sources said.

Another migrant, a 20-year-old from Eritrea, was critically injured by police during Wednesday's attempt to reach Israel.

Security sources and analysts say increased violence at the border is partly due to the increasing popularity of the route, seen as safer than risking passage by sea from Libya to Europe.

At least 19 migrants, mostly unarmed Africans seeking work or asylum in Israel, were killed by Egyptian border guards last year, while five have died this year.

Police detained another five Eritreans and three Ethiopians who tried to slip into Israel on Wednesday, the sources added.

Monthly migrant arrests by Egypt at the border surged as 2009 progressed, from just six in January to 160 in July.

Egypt has come under pressure from Israel to stem the flow, while Egypt also fears the unfettered movement of migrants in the politically sensitive Sinai border region where Islamist militants sometimes take refuge.