Germany was on terror alert as police hunted a Syrian asylum seeker suspected of plotting a bomb attack.

Police placed his flatmate under arrest and raided a contact's home after finding explosives in the fugitive's apartment.

Federal police stepped up security at airports, train stations and other "critical infrastructure," saying it was unclear whether Jaber Albakr, 22, was carrying more bomb-making material or weapons.

Albakr had allegedly been planning a bomb attack on a German airport when police moved in to arrest him in Chemnitz, about 257km south of Berlin.


But as officers prepared to burst into his flat he left the communist-era apartment block carrying a rucksack.

"We do not know where he is and what he's carrying with him. Be careful," Saxony police said.

Police questioned Albakr's Syrian flatmate, who was formally remanded in custody as a suspected co-conspirator of a "serious act of violence" after he was detained on Sunday.

Another man who knew Albakr was taken into custody for questioning today after police commandos raided his Chemnitz apartment.

Saxony police were tipped off by Germany's domestic intelligence agency that Albakr, who arrived in Germany in February 2015, might be planning an imminent attack.

Reports have linked Albakr to Isis (Islamic State) and he is believed to have consulted websites to learn how to make bombs. When specialist teams burst into the flat, they found "several hundred grams" of a highly volatile explosive, enough to cause significant damage, said Tom Bernhardt, a police spokesman.

Police have not yet confirmed what type of explosives were found, but reports said it was TATP, or triacetone triperoxide, which was used in the November 13 terror attacks in Paris and the March 22 attacks in Brussels, both of which were claimed by Isis.

Police said they fired a "warning shot" when they saw the Damascus-born refugee leave the apartment block, but he somehow managed to escape. German media were quick to denounce the failed arrest as a police "blunder".

Germany has been on edge since two Isis-claimed terrorist attacks this northern summer, in which several people were injured and both assailants died.