A growing row over United States spying is set to dominate a European Union summit in Brussels starting on Thursday after revelations German Chancellor Angela Merkel's calls may have been monitored.

EU leaders will also tackle a complex immigration crisis that was thrust into the spotlight after hundreds of migrants died in two shipwrecks off Italy and Malta this month trying to reach the continent.

The meeting is officially themed around boosting employment and the digital economy. However the two-day talks are likely to be overshadowed by mounting anger over US spying on Europe.

Merkel demanded answers from US President Barack Obama in a phone call on Wednesday night after Germany received information that US intelligence may be spying on her mobile phone.

An angry Merkel demanded "an immediate and comprehensive explanation” from Washington over what she warned was a serious breach of trust, her spokesman said in a statement.

Germany's conservative daily Die Welt called the alleged snooping "a punch in the face of German security agencies” while a Sueddeutsche Zeitung headline labeled it "the worst imaginable insult”.

The revelations will add to pressure on Brussels for new EU-wide data privacy protections, which Paris has said will be raised by French President Francois Hollande.

Ties to remain stable?

German Defense Minister Thomas de Maiziere said on Thursday that Europe can't simply return to business as usual in its relations with Washington though he stressed that ties will remain stable.

De Maiziere told ARD television the alleged surveillance would be "really bad" if confirmed.

"The Americans are and remain our best friends, but this is absolutely not right," he said.

"I have reckoned for years with my cell phone being monitored, but I wasn't reckoning with the Americans," said de Maiziere, who was previously Merkel's chief of staff and Germany's interior minister. He has been in Merkel's Cabinet since she took office in 2005.

"We can't simply return to business as usual," de Maiziere said when asked about possible effects on US-German and US-European relations. "There are allegations in France, too."

AFP — AP