Mounting ire over alleged US snooping on its allies will test Europe's unity at a summit Thursday after German and French leaders Angela Merkel and Francois Hollande demanded Washington provide an explanation.
Initially expected to be "a routine affair", according to a senior diplomat, the two-day talks from 1500 GMT between the European Union's 28 heads of state and government have been hijacked by the escalating row over covert US surveillance of its allies.
As Germany summoned the US ambassador to Berlin over suspicions Washington spied on Merkel's mobile phone -- a highly unusual step between the allies -- a French diplomatic source said she and Hollande will discuss "how to coordinate their response" on the issue.
Merkel on the eve of the summit called President Barack Obama demanding answers, warning that proof of snooping on her phone would be considered "a breach of trust".
It was Obama's second such embarrassing call this week after Hollande too picked up his phone to demand an explanation over reports of US spying on millions of phone calls in France.
Rattled by the latest exposure based on leaks from US intelligence contractor Edward Snowden, the White House has said it is not now listening in on Merkel -- but it also did not reject the possibility her communications may have been intercepted in the past.
Washington also denied reports of eavesdropping on France.
In the wake of Snowden's revelations about the activities of the US National Security Agency, several important allies have complained about US covert surveillance and the White House is struggling to stem the diplomatic damage.
The NSA affair has seen claims of US snooping on foreign leaders in Mexico and Brazil whose President Dilma Rousseff last month cancelled a state visit to Washington over the scandal.
As the row widens, the European Parliament on Wednesday asked for a key EU-US bank data-sharing deal aimed at fighting terrorism to be suspended.
But whether the EU leaders will come up with a common stand is less than certain.
'Espionage is not an EU matter'
Many governments, notably Britain and Spain, see spying as a matter of national interest firmly outside the remit of the 28-nation bloc.
"I don't imagine the (EU) Council getting into a discussion on national security," said an EU diplomat speaking on condition of anonymity.
"Espionage is not an EU matter, it's an issue of national sovereignty," said another diplomat.
At the summit, officially themed around boosting employment and the digital economy, leaders will also tackle a complex immigration crisis highlighted by this month's death by drowning of hundreds of refugees desperate to reach Europe's shores.
The two shipwrecks that saw over 400 refugees from Africa and the Middle East drown off the Italian island of Lampedusa triggered a barrage of calls for action to prevent the Mediterranean Sea from turning into what French Foreign Minister Laurent Fabius has called an "open-air cemetery".
Italian Prime Minister Enrico Letta is urging European leaders to bolster the EU's Frontex border agency and bring forward Eurosur, a planned satellite-and-drone surveillance programme to detect migrant ships in trouble.
Frontex reportedly saved 16,000 lives in the Mediterranean in the last two years but has seen its budget fall from 118 million euros ($162 million) in 2011 to 85 million euros this year due to crisis-era belt-tightening.
Malta's Prime Minister Joseph Muscat and his Spanish counterpart Mariano Rajoy have added their voices to Letta's demand for the EU to share the burden.
Italy says migrant numbers increased fourfold this year to 30,000 while Spain says the number of Africans who have tried to slip through its barbed-wire territory of Melilla in north Morocco doubled to 3,000 this year.
Analysts say it is high time for the EU to define a common policy that will address how to respond jointly to refugees from conflict and migrants in search of a better life.
"Far too many people are dying every year at the EU's external borders," said Yves Pascouau of the European Policy Centre think-tank.
With the unprecedented refugee flight from Syria's civil war, "EU leaders cannot escape answering the remaining questions any longer," he added.