Berlin: A possible terror plot on a Christmas market in Germany has been uncovered after six Syrian nationals were detained in raids by police, a German intelligence official said.

The men were detained on Tuesday following large-scale raids in four cities involving approximately 500 officers led by the Hessian State Criminal Police and the Attorney General of Frankfurt, a joint statement by the two agencies said.

The men were targets of an investigation "into suspicion of membership in a terrorist organisation and for preparation of terror attack," CNN quoted the statement as saying. According to police, the six Syrian nationals had applied for asylum and were suspected of being Islamic State (IS) members.

An attack "had not been fully planned yet" according to the statement, but the suspects were believed to have been planning to carry out attacks with "weapons or bombs on a public target in Germany".

The German intelligence official, who spoke on condition of anonymity, told CNN the suspects had been doing target reconnaissance of the areas in which Christmas markets would be held in Essen.

They would also surveilled buildings in Berlin, the official said. In 2016, a dozen people died and about 50 more were injured when a tractor trailer barrelled into a crowded Christmas market here.

The suspect had pledged allegiance to the IS in a video posted hours before the attack. He later died after a shootout with police in Italy.