German police arrested a 16-year-old Syrian at a refugee hostel near Cologne on suspicion of having contact with a supporter of Islamic State abroad and expressing his willingness to carry out a bomb attack, authorities said on Wednesday.
"He showed his readiness to commit a bombing in an internet chat," a police spokesman told a news conference in Berlin.
It remained unclear whether the teenager, who police said appeared to have been radicalised only recently, had any concrete plans for such an attack.
On Tuesday, police in Cologne had said an initial assessment had pointed to a "serious threat" but gave no further details.
"An evaluation of the young man's mobile phone proves contacts to a person living abroad who has IS links and who wanted to win over the young Syrian for Islamist activities," said the police in a statement on Wednesday.
The police spokesman said the teenager's chat partner apparently lived in the Middle East.
The father of the arrested teenager, Jamal Jairodeiah, said he hoped his son would be released soon because he believed in the German justice system.
"He's not guilty," Jairodeiah said. He said his son had not committed any crimes and somebody might have manipulated him.
Germany is on high alert after militant attacks in France and Belgium this year that have killed dozens of people and Germans are especially fearful about security risks after last year's influx of about 1mn migrants.
In July, Islamic State militant group claimed responsibility for two attacks in Bavaria.
Police arrested three Syrian members of Islamic State this month, brought into Germany by the same network that smuggled militants into France to carry out deadly attacks last November.