Two people arrested in Berlin on Thursday had links to Islamic State (Isis), which may have been planning an attack in the western city of Dortmund, a German newspaper said on Friday, citing security sources.

German police commando units arrested two men aged 28 and 46 on Thursday suspected of planning an attack and searched a cultural center, a police spokesman said.

German newspaper Tagesspiegel said the two men are from Syria and Tunisia and have links to the militant group Islamic State. The paper added that security forces suspected Islamic State could have planned a possible attack in the West German city of Dortmund from Berlin.

Criminal investigations have also been opened into other people, including asylum seekers, in connection with Thursday's arrests, the paper said.

Chancellor Angela Merkel said the security threat level in Germany remained high after the Paris attacks on 13 November where 130 people were killed.

On Thursday, an object "presumed to be hazardous" was found in a car used by the two main suspects, prompting police to block off the area and evacuate residents from several buildings, according to a police statement.

In a separate incident the following day, prosecutors in the southern German city of Stuttgart confirmed they had arrested a 34-year-old man on suspicion of arms dealing but declined to comment on a report that he may have supplied the militants in Paris with four guns.

"I can confirm that a man is in custody on suspicion of arms trading," a spokesman for the prosecutor in the state of Baden-Wuerttemberg said, adding that the arrest had been made on Tuesday. Earlier on Friday, German newspaper Bild reported that the man was suspected of selling four weapons to the militants who killed 130 people in Paris on 13 November.

The paper said four assault rifles -- two AK-47s made in China and two Zasatva M70s made in the former Yugoslavia -- were sold online by the man on 7 November to a buyer of "Arab descent".

Four emails since found on the man's smartphone indicate that he was in touch with an "Arab in Paris", Bild said. The paper added that French prosecutors believe the weapons were used in the Paris attacks.