BRUSSELS (AFP) - German Chancellor Angela Merkel, reportedly the target of US snooping on her mobile phone, said on Thursday that such conduct between friends was unacceptable.

"Spying between friends, that's just not done," Ms Merkel said as she arrived for a two-day European Union summit where the growing spy scandal has hijacked the agenda.

"I said that to US President Barack Obama when he came to Berlin and again on the phone (Wednesday)," Ms Merkel said.

"We need trust between partners and such trust needs to be re-established," she added.

Earlier Thursday, Germany summoned the US ambassador to Berlin after reports said Ms Merkel's mobile phone had been monitored by US intelligence services.

Ms Merkel called Mr Obama on Wednesday to demand an answer, warning that if the charge was proved, it would be considered a "breach of trust", according to a German statement.

The White House said it is not now listening in on Ms Merkel - but also did not reject the possibility her communications may have been intercepted in the past.

Washington also denied reports of eavesdropping on France.

Initially expected to be a routine affair, the summit of all 28 EU leaders has been taken over by the escalating row with Washington, testing ties between longstanding allies.

The EU's executive, the European Commission, pressed leaders for "a strong and united stand" as they gathered.

"Data protection must apply no matter if it concerns the emails of citizens or the mobile phone of Angela Merkel," said EU Justice Commissioner Viviane Reding.