Human Rights Watch (HRW) has expressed concerns about violent clashes in Egypt, calling upon President Mohammed Morsi to take "decisive action" against those responsible.

Reasons behind the clashes between Muslims and Christians in Khosous on 5 April are still unclear. Some sources report that Muslims were seeking revenge in response to claims that a swastika had been drawn on the wall of a mosque by Christian children.

Clashes occurred again on 7 April at the funeral procession for the four slain Christians at St Mark`s Cathedral, Cairo.

Christians came under fire as they were coming out of the cathedral. Witnesses described to HRW how hundreds of men within and outside the cathedral threw stones and Molotov cocktails, and even handguns at each other.

The Egyptian Presidency claimed it was quick to send security forces on both occasions but eye witnesses and video footage provide evidence to the contrary.

"President Mohammed Morsi needs to acknowledge the deep and longstanding problem of sectarian violence in Egypt and take decisive steps to address it before it escalates further," said Nadim Houry, deputy Middle East and North Africa director at Human Rights Watch.

"The authorities urgently need to get to the bottom of the violence at Khosus and at the subsequent funerals, and bring all those responsible to justice, promptly and fairly.

"For years people have been getting away with sectarian murder and he should break that cycle of impunity.

"Then he should reform laws that discriminate against Christians' right to worship."

According to some inhabitants of Khosous, the clashes were triggered by a dispute between Christian children playing soccer and a Muslim man and his sister, which then drew in other members of the community. It is understood some clerics encouraged the tensions.

"I heard the cleric of the mosque calling out from the loudspeaker 'Kill all the infidels' meaning the Christians," said Boules Fakhry, a shopkeeper in the area.

Violence escalated and raged for several hours, during which protagonists set fire to buildings and shops and engaged in violent clashes leading to the deaths of one Muslim and five Christians.

The Egyptian Presidency defended its response in a statement on 8 April which stated that "security forces contained the situation and deployed forces throughout the city to prevent further clashes".

Yet witness have strongly criticised the police's response.

Mina Fathy, a resident of Khosous, told Human Rights Watch "the police didn't show up until late, three CSF (Central Security Forces) trucks came, doing absolutely nothing, and things had calmed down already. They didn't say anything. They were standing 50 to 100 metres away from the church".

Among the mourners at St Mark's Cathedral was Mina Thabet who told Human Rights Watch the "situation lasted for an hour or so before the police came".

"The CSF were standing in front of the cathedral, and a few minutes later they were shooting tear gas inside of the cathedral."

The Egyptian daily Al Masry al Youm filmed a video showing members of the riot police not taking any action against men throwing stones towards the cathedral. In some footage, the men are seen apparently telling the police to shoot tear gas at the cathedral walls, with the police appearing to comply.

HRW called upon the Egyptian police "to intervene impartially and effectively in sectarian attacks".

"President Morsi should ask his police chief why the police failed to uphold the law and protect those under attack, and insist the chief take steps to ensure that the police do their job in future" Houry said.

Mr Morsi has ordered an investigation into the incidents and promised that those involved will be held accountable.