Berlin, Hamburg and Munich are believed to face the biggest terror threats in Germany, Karl Peter Bruch, the interior minister of the south-western state of Rhineland-Palatinate, has said.

Germany is on heightened alert following Wednesday's announcement that a terrorist attack was being planned for the country.

German newspaper Der Tagesspiegel reported that the United States had told Berlin that between two to four al-Qaeda militants were on their way to Germany and Britain to attempt attacks for the end of November.

Germany's interior minister on Thursday urged citizens to remain calm and not to let the recent terror alerts affect their daily lives.

"International terrorism aims to spread fear in our country. This we will not allow," Thomas de Maiziere said during a break from meeting with his state counterparts at the start of two days of regularly scheduled talks in Hamburg.

In a separate development Konrad Freiberg, the head of Germany's police union, warned on Thursday the nation of "security deficits", saying that Germany was not prepared for a terrorist attack.

"We have missed a few steps along the way and there are security deficits that we have drawn attention to," he told the Hamburger Abendblatt daily.

The statement came shortly after a suspect package containing a detonator, batteries and a ticking clock was found on a suitcase checked onto a Munich-bound plane.

The bag was detected before it could be loaded on the flight from Namibia. The flight was delayed for several hours but landed safely at Munich on Thursday morning.

Germany increased security at airports and railway stations on Wednesday in light of "concrete indications" of terrorist attacks being planned for the end of November.

Interior Minister Thomas de Maiziere said it followed a tip-off from another, unnamed country.

Germany had information on "sustained efforts" by Islamist extremists to carry out attacks, he said.

The country's threat level -- raised because of a "new situation" -- is now the same as it was during Germany's 2009 September federal elections.