Faced with persecution and an increasingly dangerous climate in Sudan in recent years, a number of Sudanese activists have fled to the Egyptian capital Cairo, hoping to continue their work in safety from abroad.
But harassment and attacks have followed them across the border, and Amnesty International has documented a series of cases of Sudanese activists living in Cairo who have faced death threats, surveillance by unidentified men, break-ins and physical assaults - including a rape and an attempted stabbing.
A group of activists - who spoke with the organization on the condition of anonymity - claimed that Sudanese Embassy and National Security Services (NSS) agents in Cairo are behind such activities, aimed at intimidating them.
"This string of reported attacks on Sudanese activists in Egypt is deeply worrying," said Audrey Gaughran, Africa Programme Director at Amnesty International.
"It appears the Sudanese authorities have widened their web of repression, with activists who fled abroad increasingly the target of threats and attacks."
Crackdown in Sudan
Amnesty International has previously documented how routine pressure and harassment from the authorities has made working on development, human rights and peace increasingly difficult in Sudan.
Human rights defenders and activists -are often targeted and subjected to arbitrary arrests and detention, sexual harassment, torture and other ill-treatment.
For some, the only way they can continue their human rights and humanitarian work is by leaving the country altogether.
But many of those carrying on their activism from Egypt are now living in fear amid the ongoing harassment, including death threats.
Among those recently targeted was a female activist who was previously tortured while in NSS custody in Khartoum.
She now lives in Egypt and told Amnesty International that, in late 2012, she was attacked and received several threatening phone calls and messages.
Facebook messages threatened she would be killed if she continued to spread "lies" about her detention and torture in Sudan. The activist also told Amnesty International that a man who made threatening phone calls twice identified himself as an officer at Cairo's Sudanese Embassy.
In December, she filed a police report after a man attempted to stab her on a street in Cairo. Although she managed to escape serious injury, the police have not yet informed her of any progress in investigating the attack. She believes the attack to be connected with her activism.
In another case, a prominent activist who has been in Cairo since 2004, told Amnesty International that he has been attacked and severely beaten twice since coming to Cairo - in 2005 and 2011.
Based on his attackers' accent and appearance, he believes they were Sudanese.
After the first attack, Cairo police were reportedly reluctant to let him file a complaint, saying it was "a fight between Sudanese people", and he had difficulty getting medical treatment for his injuries because his identification had been stolen in the attack. The eventual police report mentioned the lost identification but not the attack itself.
He continues to receive threatening phone calls every month from people who reportedly identify themselves as Sudanese officials who threaten to kill him if he does not stop his activism and "collaborating" with international organizations.
In early December 2012, two men reportedly approached a group of three activists in a Cairo street and attempted to kidnap one of them. They failed to force him into their car, but instead beat him and stole his mobile phone before fleeing.
The activist told Amnesty International that he was again attacked later that month by a group of four men who beat and threatened him, telling him that he should stop disseminating information on the conflict and the humanitarian situation in the Nuba Mountains. He received hospital treatment after this attack, and is suffering from internal injuries and a broken finger.
Based on their accents, he believes three of his attackers were Sudanese.
Three days after the second attack, a man who identified himself as a Sudanese Embassy official reportedly rang him to say the attack had been a warning, and that he would be killed if he keeps up his activism.
A female activist, who had been working on Darfur human rights issues, described to Amnesty International how she was kidnapped on a Cairo street by three men dressed in plain clothes. After forcing her into a car, the men - who she believes were Egyptian - took her to a remote location and raped her repeatedly before leaving her on a Cairo road.
The woman told Amnesty International that a few days after her rape she received a menacing phone call from a Sudanese security agent who had previously arrested her in Sudan.
He allegedly threatened her family members with further attacks.
Out of fear of reprisals, she did not report the attack to the Egyptian police.
"If Sudanese security operatives are behind these attacks, this raises serious questions about whether the Egyptian security forces are aware of such operations on their territory," said Hassiba Hadj Sahraoui, Deputy Middle East and North Africa Programme Director at Amnesty International.
"The Egyptian authorities must urgently investigate the situation, and ensure that Sudanese operatives are not carrying out operations within Egypt that threaten human rights."