(CNN) -- Leaked classified documents show the U.S. National Security Agency and its British counterpart are among the "worst offenders" of mass surveillance without oversight, according to an open letter purportedly written by Edward Snowden and published Sunday by the German magazine Der Spiegel.
The publication of the letter, titled "A Manifesto for the Truth," came just days after a German lawmaker met with the former NSA contract analyst. Snowden, who is wanted in the United States on espionage charges, offered to testify before the U.S. Congress, the lawmaker said.
"The world has learned a lot in a short amount of time about irresponsibly operated security agencies and, at times, criminal surveillance programs. Sometimes the agencies try to avoid controls," Snowden wrote, according to the news magazine.
"While the NSA and GCHQ (the British national security agency) appear to be the worst offenders -- at least according to the documents that are currently public -- we cannot forget that mass surveillance is a global problem and needs a global solution."
The letter, published in German by Der Spiegel, was written on Friday in Moscow and provided to Der Spiegel through a "locked channel," the news magazine said. It was published in German and has been translated by CNN.
Snowden, 30, has admitted in interviews he was the source behind the leak of classified NSA documents, which revealed the existence of top-secret surveillance programs that collect records of domestic e-mails and telephone calls in the United States and monitor the cell phone and Internet activity of overseas residents.
A recent report by Der Spiegel, citing documents provided by Snowden, alleged the NSA monitored German Chancellor Angela Merkel's cell phone. Some reports also suggest the United States carried out surveillance on French and Spanish citizens.
The allegations have rocked U.S.-European relations with some countries calling for investigations. It also has prompted congressional hearings in the United States, where some are calling for more transparency and more oversight of American spy programs.
The letter also accused governments of trying to squash debate about mass surveillance "with a never before seen witch hunt" that threatens journalists and criminalizes the publication of details about the programs.
"The debate they wanted to avoid is now taking place in countries around the world," the letter said.
"And instead of causing damage, the use of this new public knowledge is causing society to push for political reforms, oversight and new laws."
Snowden has been in Moscow since June after fleeing from Hong Kong. In August, Russia granted him asylum for one year.
Last week, Hans-Christian Stroebele, a member of Germany's parliament, met with Snowden in Russia. Stroebele returned from the meeting with a letter from Snowden to German authorities, which was distributed to the media.
In it, Snowden said he is confident that with international support, the United States would abandon its efforts to "treat dissent as defection" and "criminalize political speech with felony charges."
"I hope that when the difficulties of this humanitarian situation have been resolved, I will be able to cooperate in the responsible finding of fact regarding reports in the media, particularly in regard to the truth and authenticity of documents, as appropriate and in accordance with the law," he wrote.
For more from CNN, click here.