The families of six Irish students who died after a balcony collapsed during a 21st birthday party in the US have been contacted.
Charlie Flanagan, Ireland's Minister for Foreign Affairs, confirmed the news was broken to close relatives of those killed in the accident at an apartment in Berkeley, California.
"The families who have been bereaved in the tragedy in Berkeley earlier today have now all been contacted," he said.
"I again want to express my deepest sympathy to the families and loved ones of those who lost their lives in this appalling incident. We continue to provide all possible consular assistance to those affected."
The dead were at a party of students who were in the US for the summer on J1 working visas.
The balcony collapse caused the dead and injured to plunge about 40ft (12.2m) to the ground.
It is understood four died at the scene while two others died later in hospital.
At least another eight were injured, some critically, in the incident on the fourth floor of an apartment complex in the college city of Berkeley.
The accident happened at about 12.45am local time, about 8.45am Irish time.
Taoiseach Enda Kenny and President Michael D Higgins led expressions of condolences to the families, friends and loved ones, but Mr Kenny said it was too early to predict the full extent of the "dreadful accident".
He said his "heart breaks" for the families of the students involved.
The age, gender or identities of the dead are not expected to be released until later.
But all of those who were killed or injured are believed to be between 20 and 22.
There were no immediate details of how the incident occurred.
Investigations by local police in the city are centring on how many people were on the balcony when it collapsed and the stability of the structure.
The complex in Library Gardens off Kittredge Street is believed to have been built in 2006.
Addressing the parliament in Dublin, Mr Kenny said he was shocked and saddened by the incident, believed to have involved 13 Irish students aged in their 20s.
"My heart breaks for the parents who lost children this morning and I can only imagine the fear in the hearts of other parents whose children are in California this summer as they seek to contact them now," he said.
"It is truly terrible to have such a serious and sad incidence take place at the beginning of a summer of adventure and opportunity for so many young people on J1 visas in the US."
The Taoiseach said "it is too early to say (what) the full extent" is of the unfolding incident.
"According to information from local police there were 13 people on the balcony at the time of collapse, there is no official confirmation yet of their identities or the nationality of all those involved," he said.
"Current information suggests all of those are Irish."
Pictures showed the balcony appeared to have broken off from the building and partly landed on the third-floor balcony below as well as in the street.
Those caught up in the incident were rushed to hospitals and trauma centres in and around Berkeley, and also in Oakland and to Walnut Creek, with fire crews from several stations in the city involved in the emergency response.
Social workers were on hand in some of the hospitals to field calls from family and friends of those caught up in the accident.
A special emergency helpline set up at the Department of Foreign Affairs in Dublin has been inundated with calls in the wake of the tragedy.
President Higgins sent a message of condolence while on a state visit to Italy.
"I have heard with the greatest sadness of the terrible loss of life of young Irish people and the critical injury of others in Berkeley, California, today," he said.
"My heart goes out to the families and loved ones of all those involved. I have been informed of the consular assistance being provided to assist all of the families involved and I have asked to be kept informed as further details emerge."
Opposition leaders, Fianna Fail's Micheal Martin and Sinn Fein's Gerry Adams, also expressed their sympathies with the families of victims ahead of Leaders Questions in the Dail parliament.
Mr Martin said the hearts and minds of everybody were with the families of those whose "lives were taken from them in their prime in the beauty of an American summer".
The US Ambassador to Ireland, Kevin O'Malley, expressed sympathy to the families, loved ones and friends of those who died.
"All of us at the United States Embassy are greatly saddened by news of this tragic incident and are ready to do whatever we can to assist the Irish Department of Foreign Affairs at this difficult time. We will continue to co-ordinate as more details become available," he said.
Northern Ireland Deputy First Minister Martin McGuinness added his condolences over the tragedy.
"Shocking news of the deaths of several young Irish people in a tragic accident in California. My heart and sympathy goes out to their families," he said.
Archbishop Eamon Martin, leader of the Catholic Church in Ireland, said he is praying for the families of victims during their "profoundly painful" time.
"Like many others, I am shocked and deeply saddened to hear the reports of the tragic deaths of five Irish students earlier today in Berkeley, California," he said.