The family of Steven Sotloff has paid touching tribute to the murdered US journalist.

Speaking at a press conference in Florida, they said the reporter gave his life trying to find "good concealed in a world of darkness".

He wanted to give voice to those who had none, covering stories such as "the Libyan doctor in Misrata who struggled to provide psychological services to children ravaged by war; the Syrian plumber who risked his life by crossing regime lines to purchase medicine... their story was Steve's story".

Family spokesman Baral Barfi said: "Today we grieve but we will emerge stronger. We will not allow our enemies to hold us hostage with the sole weapon they possess: fear."

He added that Sotloff was "torn between his love of home life and his passion for the Arab world".

Meanwhile, President Barack Obama has vowed that the US would not be intimidated but would build a coalition to "degrade and destroy" the militant group IS.

Vice-President Joe Biden said America would follow the terrorists who posted videos showing the beheading of two journalists "to the gates of hell."

He said the American people "are so much stronger, so much more resolved" than any enemy can understand, reports AP.

Speaking at the Portsmouth Naval Shipyard in Maine, Biden said: "As a nation, we're united. And when people harm Americans, we don't retreat, we don't forget. We take care of those who are grieving and when that's finished, they should know we will follow them to the gates of hell until they are brought to justice. Because hell is where they'll reside."