Two pastors have been charged with national security crimes in Sudan, despite little evidence, giving further indication of increased persecution against Christians in the country.

The charges come despite "evidence that would not justify an arrest, let alone a conviction", according to Christian Solidarity Worldwide (CSW), a Christian charity that advocates for those facing persecution.

Rev Abduraheem and Rev Shamal were arrested in December last year, although Shamal had been released until May. Their lawyers have previously expressed concern at their conditions in the jail, stating that they are in small rooms, with limited ventilation and intermittent electricity.

The range of crimes they are charged with include espionage, propogating false news articles and waging war against the state. However, CSW says that the charges relate to just being associated with, and trying to help, the student Abdulmonem Abdumawla – who is charged alongside the two pastors.

Recently students have been targeted by the Sudanese government, and there has been violent suppression of demonstrations. Abdumawla had been raising money for a student who was badly burned during a demonstration, which appears to have drawn the attention of the authorities.

CSW said Abduraheem was arrested because he had donated money to the student's medical care, while Shamal faces charges just for being friends with his fellow pastor.

The arrests follow a number of other arrests of Christians in the country. Last year, Rev Yat Michael and Rev Peter Yen were released after similar charges had been filed against them, which drew the attention of human rights campaign group Amnesty International as well as Christian persecution charities.

Since the secession of South Sudan in 2011, Sudan's President Omar al Bashir has repeatedly called for Shariah law to be imposed in the country. He is currently indicted by the International Criminal Court for genocide in Darfur.

"We are deeply concerned to learn of the serious charges levelled against Reverend Hassan Abduraheem and Mr Abdulmonem Abdumawla simply for seeking to assist with medical expenses, and against Reverend Kuwa Shamal merely for being a Christian and a friend of Reverend Abduraheem," said CSW's chief executive Mervyn Thomas.

"These innocent men now face the possibility of a death sentence on evidence that would not justify an arrest, let alone a conviction, given its paucity."