Greenwald, a Rio-based correspondent for Britain's Guardian newspaper, also said that if Brazil wanted more data on alleged US snooping into its affairs it should offer Snowden asylum.
Snowden, a former US spy agency contractor wanted by Washington, is currently at an unknown location in Russia after Moscow granted him temporary asylum.
Brazil did not respond to a Snowden request for asylum as he sought refuge following his first explosive disclosures detailing the US government's digital dragnet.
Testifying before a Brazilian congressional panel, Greenwald accused Washington and its allies of waging a "war against journalism and the process of transparency".
"I am learning now that the United States is using this surveillance system to punish the journalistic process," said Greenwald, who, without elaborating, added he was working on material relating to France and Spain.
"We are undertaking high-risk journalism. We shall continue doing so until we publish the last document I have," Greenwald told senators investigating allegations that Washington spied on Brazil.
"I am not holding onto relevant documents nor hiding information. All that I had regarding surveillance against Brazil, and now France — I am working with French and Spanish newspapers — I publish. I don't hold onto it," he said in Portuguese.
Greenwald said governments, including Brazil's, appeared to be grateful for the disclosure of alleged US spying on them "but they are not disposed to protect the person who passes on the data".
"If the government wants information it should protect him so he is at liberty to work," Greenwald said.
"He has very limited scope to speak and runs the risk of the United States capturing him."
When he testified before the Brazilian Senate's foreign relations committee in August, Greenwald said he had personally received thousands of documents from Snowden while in Hong Kong with the fugitive.
On Wednesday, as Brazil announced it would host an Internet governance summit next year, Greenwald said: "Every time I found a document I thought ought to be published I immediately started working on it as quickly as possible to inform the public."
He added he was in almost daily contact with Snowden.
The documents released so far appear to show that the US National Security Agency intercepted Brazilian government communications, those of state-run energy giant Petrobras, as well as phone calls and emails of millions of Brazilians.
The disclosures led Brazilian President Dilma Rousseff to slam the United States in an address to the United Nations last month and scrap a planned state visit to Washington.
Citing evidence that even messages from her own office were monitored, Rousseff called the alleged spying a violation of international law.
The row deepened with allegations on Sunday that Canada, a very close US ally, also spied on Brazil's energy ministry.
Canada has mining interests in this huge, resource-rich Latin American country.
CNN reported on Thursday that Snowden's father was visiting the whistelblower in Moscow.