Frankfurt airport has denied a media report that security cameras malfunctioned in last week's attack on US troops by an alleged Islamist extremist that killed two airmen.

"The cameras here worked, and the footage recorded has been given to the investigators," Thomas Uber, a spokesman for airport operator Fraport, told AFP, in response to a report in the Die Welt daily that they had failed.

Kosovan Arid U., 21, on Wednesday shot dead the two airmen and seriously injured two others after boarding their military bus, shouting "Allahu Akhbar" ("God is Greatest") as he did so, according to German federal prosecutors.

He then allegedly put his handgun to a fifth serviceman but it jammed. Pursued by this serviceman, he fled into the terminal building where he was overpowered and arrested.

Prosecutors said on Friday that he told investigators the attack, which if confirmed as such would be the first Islamic extremist attack on German soil, was to avenge what he saw as atrocities by US troops in Afghanistan.

There are however no indications that the alleged gunman, whom investigators believe was radicalised in large part via the Internet, had links to any "international terrorist groups," prosecutors believe.

Late Thursday, the US Air Force identified the dead servicemen as Senior Airman Nicholas Alden, 25, of Williamston, South Carolina, and Airman First Class Zachary Cuddeback, 21, of Stanardsville, Virginia.

Meanwhile, the head of the German Police Union, Rainer Wendt, has called for 2,000 additional staff to monitor extremist websites.

A month ago German authorities had announced that additional security measures imposed late last year in response to indications of an imminent "terrorist" attack were set to be gradually scaled back.