* Tusk says discussion ongoing regarding scale of move
* Earlier NATO comments hint NATO permanent base unlikely
* Poland wants more US troops on soil after Ukraine crisis
WARSAW, April 5 (Reuters) - NATO will strengthen its presence in Poland within weeks, Prime Minister Donald Tusk said on Saturday, a move that could help allay fears in eastern European states for their security after Russia's seizure of Ukraine's Crimea region.
Tusk spoke three days after foreign ministers from the U.S.-led alliance ordered military commanders to devise plans for reinforcing NATO defences among its eastern European members, including Poland, a neighbour of Ukraine.
Russia's annexation of Crimea after the fall of Ukraine's pro-Russian president to mass protests has caused the deepest crisis in East-West relations since the Cold War, when most east European countries were under Soviet domination from Moscow.
But, in an apparent gesture to minimise the risk of any conflict with Russia, NATO has suggested it is more likely to beef up eastern European security through rotating reinforcements rather than permanent basing of substantial additional combat forces there.
"The strengthening of NATO's presence (in Poland), also military presence, has become a fact and will be visible in the coming days, weeks," Tusk told the broadcaster TVN. "The discussion is not about if, but rather about the scale, pace and some technical aspects of strengthening Poland's security."
Military planners have been asked to come back with detailed ideas by April 15. A NATO official said it was premature to give details as planners are still working on options.
But the measures could include sending NATO soldiers and equipment to eastern European allies for short-term reinforcements and exercises, as well as ensuring NATO's rapid-reaction force could deploy more quickly.
Poland, which spent more than four decades under the sway of the Soviet Union in a divided Europe after World War Two, is eager to see more U.S. troops based on its soil.
Polish Foreign Minister Radoslaw Sikorski said earlier this week that Warsaw would be "fully satisfied" if NATO stationed two heavy brigades in Poland. But that seems unlikely.
SPHERE OF INFLUENCE
On Thursday, NATO's secretary-general said in response to Russian concerns that what NATO was doing was in line with a 1997 cooperation pact with Russia. Under that pact, NATO agreed to defend east European members through reinforcements rather than by permanently stationing extra combat forces there.
Russia regards its fellow former Soviet republic Ukraine as within its traditional sphere of influence and has warned against Kiev's new pro-European tilt and what it perceives as a threatening sign of Western encroachment.
White House deputy national security adviser Ben Rhodes, accompanying President Barack Obama to Europe in late March, told reporters that the United States would increase rotations of ground and naval forces to NATO allies in eastern Europe, in addition to the extra planes it had already sent, and expected European NATO allies to join Washington in this effort.
Last month, responding to a request from Warsaw, the United States decided to increase the scale of its military exercises in Poland, sending to the country 12 U.S. F-16 fighter jets and 300 personnel.
Previously, there was just a small detachment of several U.S. military personnel on the ground in Poland assisting in the training of Polish pilots. The United States also has plans to deploy elements of its missile shield in Poland by 2018. (Reporting by Marcin Goettig in Warsaw and Adrian Croft in Brussels; Editing by Mark Heinrich)