The EU has warned President Barack Obama's administration of "grave adverse consequences" to the rights of European citizens from a huge US Internet surveillance programme, officials said Wednesday.

Viviane Reding, the EU's Justice Commissioner, wrote a letter on Monday to US Attorney General Eric Holder demanding "swift and concrete" answers about the spy scheme when they meet in Dublin on Friday.

She set out seven detailed questions about the PRISM spy programme, which were leaked by whistleblower Edward Snowden and revealed by The Guardian and Washington Post newspapers last week.

"Programmes such as PRISM and the laws on the basis of which such programmes are authorised could have grave adverse consequences for the fundamental rights of EU citizens," she wrote.

Her questions to Holder include whether EU citizens were targeted by the US programmes, whether Europeans would be able find out whether their data has been accessed, and whether they would be treated similarly to US nationals in such cases.

Reding said that "given the gravity of the situation" she expected "swift and concrete answers to these questions" at her meeting with Holder.

The EU official also warned that the European Parliament "is likely to assess the overall transatlantic relationship also in the light of your responses".

Obama has defended the spy programmes as a "modest encroachment" on privacy that are needed to keep Americans safe from terrorism.