Berlin (CNN) -- At least seven homemade explosives have been found along Berlin's railways within the last three days, police said Wednesday, severely disrupting traffic on several lines.
Two of the devices have exploded -- one Monday and one on Wednesday, authorities said, but nobody was killed or injured in the blasts.
Monday's explosion occurred next to a railway link between Hamburg and Berlin. The damage has not yet been repaired, authorities said.
Since Monday, at least 2,000 trains have been delayed due to the attacks, a spokesman for Deutsche Bahn told CNN Wednesday.
Another arson attempt -- this one on Berlin´s central train station -- failed Monday because officials discovered the device before it exploded.
"The German national rail company Deutsche Bahn has become the victim of extremists. Nevertheless, our controls have been successful," said Gerd Neubeck, senior manager of the company´s security department, in a written statement Tuesday.
Deutsche Bahn is offering a 100,000 euro (U.S. $138,120) reward for information leading to the arrests of those behind the blasts, according to a statement from the company.
A group called the "Hekla Welcome Committee-Initiative for More Social Eruptions" has claimed responsibility for the explosions.
"This morning we slowed down the German capital and its function as a global player in the export of armaments," said a statement from the group released Monday on a leftist website.
However, the group said it is not acting "with the intention to endanger somebody's life."
The group blames Germany´s involvement in the Afghan war for the attacks.
"German soldiers kill in different parts of the world. For 10 years, the German armed forces are at war in Afghanistan -- without the agreement of the German people," Hekla said.
At the beginning of October, a poll by the opinion research institute YouGov showed that two-thirds of all Germans do not believe in a successful military operation in Afghanistan anymore.
The alleged attackers also demanded the release of U.S. Army Pfc. Bradley Manning from jail and criticized U.S. military actions all over the world. Manning is accused of leaking thousands of classified U.S. military and State Department documents to the WikiLeaks website.
The incendiary devices found in the past few days were filled with fluid and usually placed on cables, officials said. An investigation by the regional German police agency Landeskriminalamt is under way.
"I have ordered an immediate increase in police patrols around Berlin," German Interior Minister Hans-Peter Friedrich said in a statement Wednesday.
Sources from Germany's Office for the Protection of the Constitution told CNN Wednesday that the Hekla Welcome Committee was previously unknown to authorities. The sources requested anonymity because they were not authorized to talk to the media. One source said German railways are a common target of leftist extremists. In May, a homemade explosive device hidden next to an important railway hub in the city's east caused massive delays.
In the 1970s, the leftist Red Army Faction terrorized Germany through bank robberies, kidnappings and assassinations. Its members were accused in more than 30 deaths.