Relatives of four Irish citizens being held after the stand-off at the Al-Fateh mosque in Cairo yesterday say they fear for their safety.

The three young women and teenage boy are children of Hussein Halawa, the Imam at Ireland's largest mosque in Clonskeagh in Dublin.

Omaima, 20, and Fatima Halawa, 22, left the mosque and were taken by Egyptian security forces who had surrounded and stormed the building. 

Although their mobile phones were taken, the two women managed to borrow one and called their family in Dublin, saying they had been arrested.

There had been uncertainty over the whereabouts of Somaia, 27, and Ibrahim, 17.

It is understood all four are being held at one location by Egyptian security forces.

Over 800 people have died in four days following a military crackdown on protesting supporters of ousted democratically-elected president Mohamed Mursi.

Speaking to RTÉ News, Imam Sheikh Hussein Halawa said that "every minute" counted for his three daughters and son.

He urged the Irish government to act quickly.

Minister of State at the Department of Foreign Affairs and Trade Joe Costello said the four are being detained in the city's main prison.

On RTÉ's This Week programme, the Minister said the Irish Embassy in Egypt was doing everything it could to ensure their safety.

Ambassador Isolde Moylan has been in contact with the Egyptian authorities and is trying to visit the four in jail.

Mr Costello also said they are the only four Irish citizens whose safety is at risk to come to the attention of the Department of Foreign Affairs.

He said all of Egypt and Cairo in particular is dangerous and reiterated that people should not travel to the country at present and to contact the embassy if travel is essential.

Mr Costello said Ireland would like to see peace restored in Egypt as soon as possible, for a democratic solution to be found and interim elections to take place.

Sinn Féin's foreign affairs spokesperson Seán Crowe expressed his concern for the four being held.  

"The situation in Egypt is very volatile at the moment and there is wide spread violence since the military coup," he said.

"The Irish Embassy in Cairo needs to do all they can to ensure the safety and well-being of these Irish citizens."

The siblings were among thousands who took took to the streets on Friday for what they believed would be a peaceful protest against the military.

After violent clashes, they sought sanctuary in the mosque.

The sisters and their were in their parents' native Egypt with their mother for summer holidays while their father remained at home in Ireland. Sheikh Hussein Halawa moved to Dublin with his family 18 years ago.