Six young Irish students died in the early hours Tuesday when a fourth-floor apartment balcony collapsed during a 21st birthday party in the California university town of Berkeley, authorities said.
At least seven people were injured, with some in critical condition, after the tragedy at a building located two blocks from the University of California, Berkeley campus, local media and officials said.
Most of those involved were Irish students from University College Dublin, Ireland's largest university, on the J1 work-study visa program, which allows them to teach, study, conduct research and work legally during their stay in the United States.
The Alameda County coroner's office confirmed that six people died.
"My heart breaks for the parents who have lost children," Irish Prime Minister Enda Kenny told parliament.
He said 13 people had been on the balcony at the time, citing police.
The structure collapsed shortly after police received a phone call complaining about a noisy gathering.
"It is truly terrible to have such a serious and sad incident take place at the beginning of summer of adventure and opportunity for so many young people on J1 visas in the United States," Kenny said.
He said he had spoken with US ambassador to Ireland, Kevin O'Malley, who expressed the United States' condolences.
Irish Foreign Minister Charlie Flanagan said the families of the dead had been contacted.
The victims were identified as 22-year-old Ashley Donohoe, an Irish-US dual citizen from California; and Olivia Burke, Eoghan Culligan, Niccolai Schuster, Lorcan Miller and Eimear Walsh, who were 21 and from Ireland.
Most of the victims were from University College Dublin.
"It is with deepest sadness that I speak of the tragic accident involving UCD students and their friends in San Francisco," college president Andrew Deeks said.
"Our students, like thousands of others across Ireland, head to the US each summer on J1 visas to enjoy the experience.
"It is heart-breaking to imagine that such a tragedy would strike these wonderful students when their lives are opening up to discover the world." A consular emergency response line was activated for those seeking information about loved ones.
Berkeley Police spokeswoman Jennifer Coats told AFP that officers received a call just before 1:00 am (0800 GMT).
Pictures in local media showed the balcony detached from its spot on the apartment building's exterior wall, with its railings sitting atop a balcony one floor below.
Berkeley officials said the building where the tragedy happened was finished in January 2007 and the owner of the property had been ordered to check the condition of the other balconies.
The Los Angeles Times reported that three engineers who'd visited the site said the wooden joists that once supported the balcony had been weakened by dry rot.
Thousands of Irish students arrive in the US every summer on the J1 program. A J1 visa is seen as a "rite of passage" for many Irish students, with applications from Ireland amongst the highest in the world.
California is the most popular destination and many Irish live in Berkeley, across the bay from San Francisco, when US students go home for the summer.
"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," Irish President Michael Higgins said.
"My heart goes out to the families and loved ones of all those involved." The cause of the incident was not immediately known and an investigation was under way.