Reykjavik calls for explanation of Justice Department's move to access account of politician caught up in WikiLeaks inquiry

The American ambassador to Iceland has been summoned to explain why US officials are trying to access the Twitter account of an Icelandic MP and former WikiLeaks collaborator.

Birgitta Jónsdóttir, an MP for the Movement in Iceland, revealed last week that the US justice department had asked Twitter to hand over her information. The US authorities are trying to build a criminal case against the website after its huge leaks of classified US information.

"[It is] very serious that a foreign state, the United States, demands such personal information of an Icelandic person, an elected official," the interior minister, Ogmundur Jonasson, told Icelandic broadcaster RUV. "This is even more serious when put [in] perspective and concerns freedom of speech and people's freedom in general," he added.

Iceland's foreign ministry has demanded a meeting with Luis Arreaga, the US ambassador to Reykjavík. No one at the US embassy in Reykjavík was available for comment.

Jónsdóttir is one of the site's contributors whose communications are being investigated by US authorities. A court order last week revealed that they are also seeking Twitter data from the WikiLeaks founder, Julian Assange, Dutch hacker Rop Gonggrijp and Bradley Manning, the US serviceman accused of stealing hundreds of thousands of sensitive government cables published by WikiLeaks.

The court issuing the subpoena said it had "reasonable grounds" to believe Twitter held information "relevant and material to an ongoing criminal investigation". A Twitter spokesman said it was company policy to inform users if there was an investigation when legally possible.

WikiLeaks has demanded that Google and Facebook reveal which of their users are under similar scrutiny.

Jónsdóttir told the Guardian: "I am very pleased that we are going to have this meeting. There are a few things here that need to be straightened out and I am very grateful that this issue is being treated as seriously as it should be."

She said she was talking to lawyers at the Electronic Frontier Foundation and would attempt to stop the justice department's move.

Most messages on Twitter are public but users can send private messages. The court order is also seeking details of source and destination internet protocol addresses used to access the accounts. These would enable investigators to identify email addresses and how the named individuals communicated with each other.

The US attorney general, Eric Holder, has said he believes Assange could be prosecuted under US espionage laws. Holder said the leaks had endangered US national security. "The American people themselves have been put at risk by these actions that I believe are arrogant, misguided and ultimately not helpful in any way," he said.

• this article was amended on 18 January 2011. The original referred to the Electronic Freedom Foundation. This has been corrected.