By Jonathan Beale
BBC News, Washington
US Secretary of State Condoleezza Rice has worked hard at repairing America's relations with Europe following the Iraq war. But once again the strains are showing.
Ms Rice is expected to deliver a robust defence of US actions
Her first meeting with Germany's new Chancellor Angela Merkel on Tuesday had been meant to mark a dramatic improvement in US-German ties.
But Germany's opposition parties are demanding an investigation into claims that more 400 CIA flights used German airspace. The suspicion is that some may have been used to transport terrorist suspects to secret prisons for interrogation.
The issue will follow Ms Rice as she moves on to Bucharest later in the week. Romania is one of the countries that human rights activists believed housed a CIA secret prison - a charge the Romanian government denies.
Questions will continue to pursue her from Kiev to Brussels.
The European Commission has warned that any member guilty of breaking international law risks losing voting rights.
Britain - which currently holds the EU presidency - has asked the US for "clarification".
The question of CIA secret flights and secret prisons will dog her every move.
Germany is the latest EU country reported as a landing spot
Ms Rice has promised to give Europe a "comprehensive" response before she leaves for Europe on Monday.
But it seems highly unlikely that she will answer the charges directly.
Speaking on Sunday, President Bush's national security advisor Stephen Hadley said that if such operations were taking place "they're the kind of things that one cannot talk about".
Mr Hadley went on to say "...the information would help the enemy... it would put countries who are co-operating with us at risk", all the while refusing to confirm or deny the reports first published in the Washington Post.
Instead, Ms Rice is expected to deliver a robust defence of America's treatment of prisoners.
She will restate that America does not transport prisoners to be tortured, that it respects international law and the sovereignty of other countries, and that its co-operating with its European allies in fighting the threat of terrorism.
In other words - "we're all in this together".
Allegations have potential to damage Europe as well as America
While the US will continue to face questions about whether secret prisons exist, European governments will also be pressed on how much they knew.
The US assertion that it respects the sovereignty of other nations suggests that it would have informed their allies of their activities - at least to some extent.
Tom Malinowski, Washington-based Advocacy Director of Human Rights Watch, says Ms Rice is in an impossible position.
He also questions the wisdom of the European Commission to threaten sanctions against any country that may have housed secret prisons.
In his view, the EU should be doing the opposite - encouraging members states to come clean.
'The biggest loser'
But it is the US and its image that will suffer the most.
US reassurances that it does not allow the torture of prisoners will count for little if no-one can see what is really going on.
Prisoner abuse at Iraq's Abu Ghraib jail took place - even though the administration has condemned wrongdoing.
Once again the world will question America's moral authority.
Inevitably, Ms Rice will be asked how can democracies allow people to disappear and then lecture the rest of the world on human rights.
She believes she can provide an answer and reassure America's allies. But it's hard to see how, if she refuses to answer specific allegations.