As war looms between Sudan and South Sudan, Christians of southern origin living in Sudan fear retribution from its Islamic government.

As of April 8, at least half a million ethnic southerners (the majority of whom are Christian) living in Sudan are now considered foreigners if they have not registered for citizenship. Officials in Khartoum, Sudan’s capital, gave southerners another 30 days to register or leave the country.

Most of those affected were refugees that fled north during the long civil war between the mostly Islamic north and the largely Christian south. The war, which ran from 1983 until the signing of a peace deal in 2005, killed nearly 2 million people. Most ethnically southern Sudanese living in Sudan have no strong ties to South Sudan, AllAfrica reported.

However, Compass Direct News reported Thursday that the Sudanese government has cut off all flights and land routes to South Sudan, trapping southerners in the north. Those attempting to board planes bound for Juba, capital of South Sudan, were turned away after officials said they required documents from Juba in order to leave.

Tensions have been escalating over the control of oil fields located along the disputed border between the two countries. South Sudan seceded peacefully last July, taking 80 percent of Sudan’s oil in the split. But now the two countries have resumed fighting.

The BBC reported last week that both sides have ceased negotiations. The Los Angeles Times reported Friday that Sudanese President Omar Hassan Ahmed Bashir declared Sudan would give South Sudan a “final lesson by force. We will not give them an inch of our country, and whoever extends his hand over Sudan, we will cut it.”

Compass reported that Christians in Khartoum have already faced threats by Muslims in the area and that many Islamic groups are calling for the deportation of ethnic southerners. On April 9, an Islamic mob threatened to demolish a Bible school with a bulldozer, but police managed to send them away.

Christianity Today has previously covered the religious violence in Sudan, including a report on South Sudan’s vote for independence in early 2011 and a report on the signing of the peace agreement in 2005.