Indie rock band Editors called on Theresa May and world leaders to do more to help Syrian and Afghan refugees as they visited families who have fled their homes and are living in camps.
The band recently visited two camps in northern Greece and met displaced families and said they were deeply moved by what they saw.
Speaking from Athens airport, lead singer Tom Smith told the Press Association: "At times it was hard, very hard, but I'm still glad we went. It was emotional at times to hear these stories face to face and to sit with families and children.
"We met one lady who invited us into her tent and we sat with her and heard what had happened to her family, it was impossible not be to moved.
"She tried to cross the border into Turkey six times and not managed, she had paid smugglers and when she got through she was separated from her husband and two of her children.
"She didn't know what had happened to them for six months until she found out they made it to Germany but she had no idea when she will see them again. The frustration was quite apparent and moving. That unbearable frustration, I found that hard."
Smith urged Mrs May to commit to action when she meets world leaders in New York next week.
He said: "Greece is on it's arse.
"The vast majority of refugees are being sheltered by underdeveloped countries, the richer countries are doing the least.
"The UK needs to keep its promise. It said it would house 20,000 and it is nowhere near that. That would be a start. And do more to help these families, help reunite those that have split up."
Smith said the band visited one camp made up of tents but also visited another set up in an old orphanage in a small town.
He said: "It felt part of a town. We met the lady that runs the place and she said there had been no problems with the community, they had been welcoming.
"That was quite amazing and inspirational. That made you think if it was a small town in the UK, how would that go down? Part of me believes the best but the other part would be worried."
Smith said the band were invited to visit the camps by Oxfam after contributing a song to the charity's Glastonbury CD to support its work with refugees around the world.
"We didn't go with any great political knowledge of the situation but as human beings and this kind of problem speaks to me on that level, people who have had to abandon their homes, " he said.
Smith will be joining the public march in central London on Saturday, demanding action to support refugees.
He said: "I'm looking forward to getting stuck in with my kids."