By Michael Nienaber

BERLIN (Reuters) - Germany is looking into a claim by Islamic State that one of its followers was responsible for a fatal stabbing in the German city of Hamburg two weeks ago in which a young couple were attacked.

A spokesman for the Federal Public Prosecutor said on Sunday investigators were checking the claim in a statement published by the militant group's Amaq news agency over the weekend.

"We are looking into this and are evaluating the credibility of the content," the spokesman said. He noted that it appeared a bit unusual that the claim was published two weeks after the attack. "This differs somewhat from the pattern," he said.

The Federal Public Prosecutor's Office, which is responsible for investigating crimes concerning national security, will decide in coming days if it has grounds to take over from local authorities, he added.

In the attack, an unknown perpetrator set upon a young couple sitting under a bridge at the Alster river in central Hamburg. He stabbed the 16-year-old man repeatedly from behind and kicked his 15-year-old female companion into the river.

The stabbing victim died of his injuries shortly afterwards in hospital while the young woman managed to escape.

In its claim of responsibility, the group said: "A soldier of the Islamic State stabbed two individuals in Hamburg city on the 16th of this month. He carried out the operation in response to calls to target the citizens of coalition countries."

A police spokeswoman in Hamburg said local investigators were checking the statement and its credibility, but she added that the claim of responsibility raised a number of questions.

"At this point, it is important to point out that the murder squad is still investigating in all directions," she said.

In a statement issued shortly after the attack, police had said the attacker was of "southern appearance", aged between 23 and 25 years, with stubble. They added the background of the attack was unknown.

Experts say it is not clear how close the connection is between groups and individuals proclaiming allegiance to the Islamic State militant group.

The Islamist organization is increasingly under pressure from regional and international forces in its Middle East heartland of Syria and Iraq.

In July, the group claimed responsibility for two attacks in the German state of Bavaria - one on a train near Wuerzburg and the other at a music festival in Ansbach that wounded 20 people.

In mid-October, police arrested a Syrian refugee suspected of planning a bomb attack on an airport in Berlin. The 22-year-old man committed suicide in prison shortly after his arrest.

Officials said Jaber Albakr spoke to a member of Islamic State in Syria by telephone about a possible target a day before German police discovered explosives in his apartment.

(Reporting by Michael Nienaber; Editing by Susan Fenton and Alexandra Hudson)