* France, Germany call for EU to take over borders
* Measure reflects mounting frustration with Greece
* EU executive to present border guard plan on Dec. 15
* EU leaders to tackle migration at summit on Dec. 17
By Alastair Macdonald
BRUSSELS, Dec (Reuters) - Germany and France have proposed giving an EU border force the power -- in theory at least -- to patrol Greece's frontiers uninvited by Athens in the latest sign of hardening attitudes towards solving Europe's migration crisis.
The proposal, in a letter sent last week to the EU executive in Brussels and seen by Reuters on Tuesday, would in principle apply to any member state, not just Greece. But it is driven by frustration that Greek failure to control large numbers arriving by sea is putting the EU's open-borders Schengen zone at risk.
EU leaders, struggling for unity and facing competing pressures at home, will again discuss the crisis at a summit on Dec. 17. Diplomats expect calls for more coercive pressure on both governments and migrants to follow policy set in Brussels.
A draft of conclusions for the summit speaks of "rapidly" fixing failings in border control and of "measures to discourage refusal of registration" by migrants -- the closest official EU language has come to calling for the use of force and detention.
Talk of "Europeanising" frontier defence is also growing.
"In exceptional circumstances, Frontex should be able to deploy rapid reaction teams to the frontiers on its own initiative and under its own responsibility," the French and German interior ministers, Bernard Cazeneuve and Thomas de Maiziere, wrote to the European Commission last Thursday.
Frontex is the EU agency that coordinates border management.
The Commission is likely to put forward such a plan next week, EU officials say, as part of a package of measures on Dec. 15. It would include a new European Border and Coastguard Agency which could be deployed without a request from the state in question. Frontex currently needs an invitation.
The proposals by Cazeneuve and de Maiziere, who also said Italy and Greece must keep all migrants in "reception centres" for "as long as necessary" to check their claims, were made on a day when Athens was under huge pressure to invite Frontex forces in or face being effectively suspended from the Schengen zone.
The Greek government finally agreed to accept EU help on the eve of a meeting of EU interior ministers on Friday.
However, Prime Minister Alexis Tsipras, a leftist who has had tumultuous relations with the EU over Greece's debt crisis and who rules in coalition with right-wing nationalists, insists that national law prevents Frontex patrolling Greek frontiers.
Athens has also been sceptical about cooperation with Turkey following an EU deal with Ankara to help keep Syrian refugees in Turkey. The focus of EU concern is the sea between Greece and Turkey where naval patrols might be feasible without the level of Greek cooperation that is needed for Frontex to work on land.
COERCION
Italy, too, has made clear its reservations about calls from other EU governments for tougher frontier controls.
But the arrival of about a million migrants this year, mostly by sea via Greece and Italy, who have trekked north over Schengen's open borders, has piled pressure on leaders such as French President Francois Hollande and German Chancellor Angela Merkel.
Last month's Islamic State attacks in Paris, in which some of the assailants may have arrived from Syria via Greece, has also increased pressure for action on the EU's frontiers, as Cazeneuve and de Maiziere spelled out in their letter.
Donald Tusk, the former Polish prime minister who will chair next week's summit, urged governments last week to be ready to detain all migrants for "as long as needed" to check their identities and claims. He said it could be for up to 18 months.
The French and German ministers echoed that by saying that reception facilities in Italy and Greece should hold all people until a decision was made to deport them or let them apply for asylum, either locally or "relocalised" elsewhere in the EU.
"All migrants arriving in Italy and Greece must go through the reception centres," they wrote. "That means these centres must be able to take in enough people and hold them for the time required ... so that the necessary security checks can be made and their status clearly established, between relocalisation, asylum claim in Greece or Italy, or deportation."
A French government source said the letter was not intended to promote new EU measures but to stress a need to implement what has already been agreed. The source said: "It's a political push that says: 'We're now at the time for making decisions and this has to be implemented very quickly'."
EU officials say they are anxious to see Greece implement measures to contain migration, including in cooperation with Turkey, while winter weather reduces the numbers crossing, so that next spring does not see a repeat of this year's chaos.
One senior EU official voiced frustration with the refusal of Greece and Italy to force new arrivals into reception centres. Athens and Rome insist they will not run "concentration camps".
"As long as there is no detention, you cannot do security checks. So it cannot be optional," the EU official said.
"It's not going to be a nice discussion, but you have no other solution. There's an obligation on all of us to check who is entering Schengen."
Responding to the position of the United Nations and other agencies which say detention of migrants should be avoided where possible, another senior EU official told Reuters the bloc would have to "push the limits of international law" to oblige people to stay put while their claims were assessed.
"If we don't keep people in some kind of centre until we put them on a plane," he said, "then it doesn't work." (Additional reporting by Karolina Tagaris in Athens, Steve Scherer in Rome and Jean-Baptiste Vey in Paris; Editing by Gareth Jones)