Officials have refused to hand over dozens of German intelligence documents detailing the extent to which the country’s spy agencies cooperated with their U.S. counterparts.

Experts say the government’s reluctance to fully inform Parliament stems from a fear that leaks could imperil the flow of vital intelligence information from the United States.

But the impasse also underscores differences between European and American systems of intelligence oversight. In the United States, Congress wields significantly more power to demand documents from intelligence agencies than do many European parliaments.

The top Republicans and Democrats on the congressional intelligence committees are privy to many of the deepest American secrets, which is not always true of European parliamentarians — particularly those from the opposition.

The German government was among the most vocal critics of the NSA’s electronic eavesdropping programs after reports emerged last year that its targets included Chancellor Angela Merkel’s cellphone and the electronic communications of millions of ordinary Germans. Lawmakers from the governing coalition backed the creation of a parliamentary inquiry into the NSA’s activities in Germany earlier this year.

But since taking up its work, the panel has been told it can only see certain documents in redacted form, while others need to be reviewed by Germany’s allies — particularly the U.S. and Britain — before they can be released to lawmakers.

This “consultation procedure” has riled opposition lawmakers, who say it is a major obstacle to their work.

“My voters ask me: tell me what’s actually happening with the NSA,” said Green party lawmaker Hans-Christian Stroebele. “I need to be able to say I’ve done everything possible to find out. But I can’t because the files held by the German government aren’t being handed over.”

Responding to a question from The Associated Press, a spokeswoman for Merkel said Friday that the consultation was regulated by Germany’s secrecy accords with other governments, but rejected earlier suggestions by government officials that this amounted to a U.S. veto.

“The German government has the final word,” Christiane Wirtz insisted.

A senior U.S. intelligence official, who spoke on condition of anonymity because he wasn’t authorized to discuss the issue publicly, said the U.S. believes it has the right to review and comment on any information turned over to the German Parliament, and to warn of possible consequences if sensitive material is shared with lawmakers who might leak it to the media.

A similar consultation would take place with Berlin if Congress wanted access to documents that involved sensitive information received from Germany, the official said.

Veto or not, Germany may be wary of overriding the wishes of an ally such as the United States, said Christian Tuschhoff, an expert on trans-Atlantic relations at Berlin’s Free University.

“I think they feel it’s more important to protect our American friends and keep channels of intelligence communication open,” he said, adding that there’s also the issue of whether opposition lawmakers could be trusted with sensitive information. “The government fears secrets will spill out into the media.”

This appears already to have happened in a way that has called into question the German government’s assertion that its intelligence cooperation with the United States — ramped up after the Sept. 11, 2001, terror attacks — was legal under German law.

Earlier this month, public broadcaster ARD and Munich daily Sueddeutsche Zeitung cited information given to the panel to report that Germany’s foreign intelligence agency illegally funneled raw data from De-Cix, a major Internet data junction in Frankfurt, to the NSA between 2004 and 2008. The government had previously denied that data belonging to Germans were collected and sent to the NSA; this week it warned that any lawmaker who leaked information to the media would be prosecuted.

Martina Renner of the opposition Left Party said fact that intelligence agencies can vet which documents lawmakers can see is undemocratic. “It’s incomprehensible that the very people we are investigating are also the people who decide which documents we’re allowed to see.”