Ireland's Taoiseach Enda Kenny has said his "heart breaks" for the families of five students who have died in the US after a balcony collapsed during a 21st birthday party.
Addressing the parliament in Dublin, Mr Kenny said it was too early to know the full extent of the tragedy involving Irish 20-somethings on a working holiday in Berkeley, California.
At least another eight were injured, some critically, in the incident on the fourth floor of an apartment complex in the college city in the early hours of the morning.
The balcony collapse caused the dead and injured to plunge about 40ft (12.2m) to the ground.
Mr Kenny said he was "shocked and saddened" to hear of the unfolding events.
"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 "dreadful accident".
"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."
All of those who were killed or injured are believed to be aged 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.
The complex in Library Gardens off Kittredge Street was built in 2006.
Pictures showed that 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.
The accident happened after midnight local time, eight hours behind UK and Ireland time.
Relatives of the dead and injured have been in contact with at least three hospitals in and around Berkeley where emergency trauma centres were set up following the accident.
Berkeley Fire Department and police and city officials were on the scene, which has been sealed off, while eyewitness reports said a number of bodies remained outside the apartment.
Ireland's Foreign Affairs Minister Charlie Flanagan confirmed the tragedy involved Irish students who were in the US state for the summer on J1 working visas.
"It's an appalling tragedy, an appalling loss of life for young people whose hopes and dreams of the future have suddenly and without notice been shattered," he said.
"My heart goes out to the families and loved ones of the deceased and those who have been injured in this appalling accident," said Mr Flanagan.
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.
At least four people were taken to the Sutter Alta Bates Summit Medical Centre.
Irish President Michael D 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.
"This devastating news will be felt by Irish people everywhere, and particularly by those who have family and friends abroad."
Archbishop Martin said he is also praying that the injured students will recover.
He added: "The death of a child or young person is the worst news that a parent can receive.
"At this profoundly painful time I pray for those who have died, and ask also for prayerful support for their grieving families, fellow students and loved ones."