By Noah Barkin

BERLIN (Reuters) - German Chancellor Angela Merkel and fellow European leaders are pressing the new British government to trigger divorce proceedings with the European Union as soon as possible.

But behind the scenes, senior German officials who spoke to Reuters on condition of anonymity due to the sensitivity of the issue, say they fear a swift move by London to invoke Article 50 of the EU treaty risks creating an impossibly short window for negotiating Britain's departure.

Further complicating the task, EU leaders have rejected the possibility of any negotiations before Britain moves on Article 50, a step which would start a two-year countdown to Brexit.

Behind their stance is a desire to send a message to Britain that it cannot hold the EU hostage by horse trading on the terms of an EU exit before it commits to leave.

But six top officials in Berlin and Brussels described this position as problematic, with one dismissing it as "absurd".

Some believe Europe's hard line on the sequencing of Brexit talks will need to be revised, perhaps as early as October, when new British Prime Minister Theresa May is due to attend her first meeting of EU leaders in Brussels.

The comments reveal the depth of anxiety in Europe's key capitals about how both sides in the Brexit showdown have positioned themselves in the weeks after the shock June 23 vote to leave the bloc.

"It was not wrong to send a tough message after the Brexit vote but I don't think the current stance is sustainable," said one official. "You need to start some sort of process as soon as possible, whether you call it negotiations or not."

A second senior official said: "It's absurd to think that we won't negotiate on anything before Article 50 is invoked."

May, on her first foreign trip since replacing David Cameron as prime minister last week, visited Berlin on Wednesday for talks with Merkel before traveling to Paris on Thursday to discuss Brexit with French President Francois Hollande.

At a news conference in Berlin, she said Britain needed time to agree on its objectives for the talks and would not trigger Article 50 this year. Merkel said it was understandable that Britain would take a few months to figure out its negotiating strategy, but added: "Nobody wants a prolonged period of limbo."

The French have taken a tougher line, pressing Britain to move fast, and launching an open campaign to woo London-based financial firms to Paris.

FAR TOO SHORT

Behind the concern of the German officials is a creeping realization that the two-year window for negotiating a Brexit, as set out in Article 50, is far too short.

An extension of the period is possible, but it would require the unanimous agreement of the remaining 27 EU member states, and is therefore seen as unlikely, or at best unsure.

Berlin is also skeptical about the possibility of Britain revoking Article 50 once it has been triggered.

This means that something will have to give, German officials say. They spell out two possible scenarios.

Under the first, the EU would revise its position and agree to a prolonged period of negotiations before Article 50 is invoked. That would win both sides extra time before the clock starts ticking, but it would represent a climbdown and probably provoke outrage in some EU capitals, notably Paris.

The second option, in the event May triggers Article 50 early next year, would be for Britain to settle for a very basic framework for its future ties with the EU, based on an existing model similar to that of Norway or Switzerland.

Even then, the deadline of two years is widely viewed as a stretch.

A third senior official said it took the EU three years to seal its divorce from Greenland, a negotiation that was focused almost exclusively on fishing rights.

That official estimated that the EU and Britain, because of the complexity of their relationship, needed at least twice that time -- six years -- to seal their separation, describing two years as "mission impossible".

Adding to the muddle is the heavy election calendar in Europe next year, which officials fear could lead to paralysis.

Germany, France and the Netherlands are all holding elections in 2017, Spain is still struggling to form a government after two inconclusive votes, and Italian Prime Minister Matteo Renzi has said he will resign if he loses a referendum on constitutional reform in the autumn.

Leaders in these countries will be focused on their campaigns. If there are changes in power, new governments will need time to settle in.

"Do you really think that Europe will be in a position to focus on Brexit talks next year if its five biggest countries are in the middle of elections or dogged by political uncertainty?" a senior Brussels-based official said.

DOWN TO EARTH

Even if formal negotiations do not start for half a year or more, the official said it was important that Britain and the EU converge on a "corridor of principles" for Brexit talks in the months ahead.

One of the big worries in Berlin and other capitals is that London has unrealistic expectations about what it can secure from the Brexit negotiations, particularly on the tradeoff between access to the EU's single market and respect for the bloc's core principle of free movement.

British diplomats also acknowledge that the team May has put together to steer Brexit talks has a starry-eyed view of what concessions London can win from the EU and say their European counterparts need to deliver this message to them directly.

"A lot of people have 'climbed up trees', people like David Davis and Liam Fox," said one of the German officials, referring to the new Brexit and trade ministers in London. "They need time to climb down."

The problem is not just on the British side. It remains unclear who will take the European lead in negotiations, although officials say the aim is to clear this up by September.

Berlin is reluctant to hand over responsibility to the executive European Commission, its president, Jean-Claude Juncker, and his chief of staff Martin Selmayr, out of fear they could take an overly confrontational stance toward Britain.

An alternative would be to let the European Council, led by Poland's Donald Tusk, take the lead, with input from sherpas in European capitals and with the Commission playing a secondary, supportive role.

Either way, agreeing a common negotiating strategy between the remaining 27 EU countries is becoming a huge challenge, especially because Berlin and Paris have different views about how to treat Britain during the process.

(Editing by Timothy Heritage)