The US government is refusing to grant Angela Merkel access to her NSA file or answer formal questions from Germany about its surveillance activities, raising the stakes before a crucial visit by the German chancellor to Washington.

Merkel will meet Barack Obama in three weeks, on her first visit to the US capital since documents leaked by whistleblower Edward Snowden revealed that the NSA had been monitoring her phone.

The face-to-face meeting between the two world leaders had been intended as an effort to publicly heal wounds after the controversy, but Germany remains frustrated by the White House's refusal to come clean about its surveillance activities in the country.

In October, Obama personally assured Merkel that the US is no longer monitoring her calls, and promised it will not do so in the future. However, Washington has not answered a list of questions submitted by Berlin immediately after Snowden's first tranche of revelations appeared in the Guardian and Washington Post in June last year, months before the revelations over Merkel's phone.

The Obama's administration has also refused to enter into a mutual "no-spy" agreement with Germany, in part because Berlin is unwilling or unable to share the kinds of surveillance material the Americans say would be required for such a deal.

Merkel is intensely aware of the importance of the surveillance controversy for her domestic audience, and is planning to voice Germany's concerns privately with White House officials and leading senators. She will also be "forthright" in confronting the issue if she is asked by reporters during a press conference with Obama, according to a well-placed source with knowledge of the trip.

A senior US administration official denied the surveillance controversy would overshadow Merkel's visit.

"Given that we already have a dialogue on these issues in intelligence and diplomatic channels, and given the other critical issues the two leaders will need to discuss, including T-TIP [trade negotiations] and Ukraine, I would not expect NSA issues to be a big part of the discussions between Chancellor Merkel and President Obama," the official said.

Details of Merkel's visit have not been publicly announced, but the Guardian understands she will arrive in Washington on 1 May, for a two-day trip, giving a speech to the American Chamber of Commerce and meeting with the head of the IMF, Christine Lagarde.

However, the main purpose of the trip will be Merkel's meeting with Obama at the White House on 2 May, an encounter immediately followed by a joint press conference with the two world leaders.

The latest information about the US refusal to divulge surveillance information about Merkel was revealed by in response to a parliamentary query by Green MP Omid Nouripour, who asked if the German chancellor had requested the release of paperwork relating to US intelligence agents' surveillance of her phone calls.

In its response, which is believed to have been released some weeks ago, but which only recently surfaced in public, a spokesperson for the German interior ministry confirmed that Merkel's government had submitted an official request on 24 October, but that the US government "had not supplied information in this regard".

Two weeks ago, the German magazine Der Spiegel said the NSA kept more than 300 reports on Merkel in a special databank concerning heads of state.

The report, published in partnership with The Intercept, a website set up by the former Guardian journalist Glenn Greenwald, was based on documents provided by Snowden. Previously, Der Speigel revealed the NSA had monitored Merkel's mobile phone for as long as 10 years.

Nouripour, who is the Green party's spokesperson on foreign affairs, said he intended to make further inquiries with the government and would seek to clarify if Merkel had asked for her NSA file to be destroyed.

Nouripour criticised both the German and the US governments for their response to the NSA revelations. "Last year, their failure to answer questions could have been due to genuine ignorance – now it looks like deliberate obfuscation. The Germans aren't asking the tough questions so they can protect their notion of a transatlantic partnership, and the US is happy that the Germans aren't asking tough questions so they can avoid further diplomatic scandals."

The news comes amid growing German frustration with the US and UK governments' failure to yield basic information about their surveillance activities. Earlier this week, interior minister Thomas de Maizière told Der Spiegel that the US response to the affair remained "inadequate".

"If two-thirds of what Edward Snowden reports, or of what is reported with attribution to him, is correct, then I come to the conclusion: the USA is acting without any restraint," said de Maizière, who emphasised that he was still a "transatlanticist by conviction". "America should be interested in improving the current situation. And words alone won't achieve that."

The White House national security spokeswoman Caitlin Hayden said Merkel and Barack Obama "spoke on the phone in late October last year" about the NSA's monitoring of her calls.

Hayden reiterated that Obama assured Merkel that the US was no longer listening to her calls and had committed not to do so in the future. She added: "I'm not going to get into the detail of those discussions and exchanges, which are ongoing."

The US government's refusal to allow Merkel access to her own file contrasts with the relative ease with which German citizens are able to access files relating to the surveillance activities of the East German secret service, the Stasi.

In January 1992, after pressure from human rights activists, the German government took the unprecedented step of opening up the Stasi archive to the public – the federal agency in charge of the Stasi archives still receives around 5,000 applications a month.

In 1992, 13,088 pages worth of files relating to the NSA's surveillance of the West German government, sold to the Stasi by the US spy James W Hall, had been returned to the US, with permission of the German interior ministry.

Angela Merkel has defended the decision to keep access to the Stasi archive open to German citizens, and has reportedly used the opportunity to view her own Stasi file in person. "Many in former socialist countries envy us for this opportunity", she said in 2009.

In Germany, the aftermath of the Snowden revelations continues to be debated with vigour. On Wednesday, the head of a parliamentary inquiry into NSA surveillance resigned over a disagreement as to whether Snowden should be invited as a witness. Green and left politicians insist that the whistleblower should be invited to give testimony in person, but panel chairman Clemens Binninger, of Merkel's Christian Democrats, was more sceptical, arguing that most of the key information was already out in the public realm.

Academics at Rostock University, meanwhile, have voted to award Edward Snowden an honorary doctorate. Members of the philosophy faculty said they wanted to reward Snowden's "civil courage" and his "substantial contribution to a new global discourse about freedom, democracy, cosmopolitanism and the rights of the individual".