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* ELN, FARC have stepped up offensive in recent weeks
* ELN has sought participation in FARC peace talks
* Rebel groups weakened but still formidable (Adds detail, background)
By Annika Breidthardt and Helen Murphy
BERLIN/BOGOTA, Feb 5 (Reuters) - Two German men kidnapped by Colombia's ELN rebels in a sparsely populated jungle area near the Venezuelan border are retirees travelling as tourists, Germany's Foreign Ministry said on Tuesday, after their captors suggested they were spies.
The National Liberation Army, Colombia's second-biggest guerrilla group, said on Monday it had captured two men believed to be from Germany and considered them to be intelligence agents because the pair could not explain why they were in the area.
"We have to assume that two German citizens are being held against their will in Colombia," a spokeswoman for the German foreign office said on Tuesday.
"The German citizens are pensioners who were travelling in the region as tourists," she said. The ministry has set up a crisis group to deal with the abduction, she said.
The ELN, considered a terrorist group by the United States and European Union, said in a statement the Germans had been seized in Catatumbo, near the border with Venezuela, and identified them as Uwe Breuer and Gunther Otto Breuer.
The rebel group is not included in the negotiations under way in Cuba between the Colombian government and the nation's biggest insurgent group FARC, or the Revolutionary Armed Forces of Colombia, to bring an end to five decades of war.
The ELN, which may be stepping up pressure on the government so it can be included in the talks, also seized a Canadian and two Peruvian gold-mine workers in northern Colombia last month.
The kidnapping of foreigners, including oil workers and tourists, had been a common method of pressuring the government over the past decade and of keeping people from venturing into rebel-controlled areas.
But military pressure and widespread anger from Colombians and the international community seemed to slow the trend until recently. It is not known how many foreigners remain in rebel camps.
The FARC on Saturday pledged to release three members of the armed forces it kidnapped last week.
For more than a decade, U.S.-backed strikes against the FARC and ELN have severely weakened the rebels and limited their ability to attack the country's economic drivers, helping attract billions of dollars in foreign investment.
But an escalation of violence in recent weeks has left scores of insurgents and government troops dead, demonstrating that even while the groups are weakened, they are by no means spent.
The ELN group has battled a dozen governments since it was founded in 1964. Inspired by the Cuban revolution and established by radical Catholic priests, it was close to disappearing in the 1970s but has steadily rebuilt its strength since then.
Both the ELN and FARC have stepped up attacks on Colombian infrastructure this year and last, repeatedly hitting oil pipelines and power lines. (Reporting by Thorsten Severin; Writing by Annika Breidthardt; Editing by Pravin Char and Cynthia Osterman)