* Unclear whether this withdrawal or conscript rotation
* U.S. has said 40,000 troops are on border with Ukraine
* Russia says they are there for military exercises
* Kerry has called for Russian troop withdrawal (Adds background, quotes, Foreign Ministry comment)
KIEV, March 31 (Reuters) - The number of Russian troops deployed on the border with Ukraine is decreasing, a Ukrainian Defence Ministry official said on Monday, but he cautioned that this might not represent a pull-back of forces.
Major-General Oleksandr Rozmaznin told journalists the drop in numbers might reflect a scheduled rotation of conscripts rather than a withdrawal, and he would not confirm the numbers involved.
"The number of soldiers has fallen, but the exact figure is pretty approximate. I can not confirm the number," said Rozmaznin when asked if as many as 10,000 soldiers might have left the border area.
"The number has definitely dropped and it (the situation) has calmed down. We won't rejoice and shout 'Hurray'. It doesn't matter how many of them there are, we just need to make sure our defences are strong," he said.
Conscripts made up a large part of the Russian army and "a certain number of conscripts could be stood down and swapped for others", Rozmaznin said.
The Russian military build-up on Ukraine's eastern and southern borders, as Russian forces moved into Crimea and Russian President Vladimir Putin warned that Moscow reserved the right to defend Russian-speakers in Ukraine, caused alarm in Kiev at the prospect of a broader military invasion.
Russia says the build-up represents military exercises and Russian officials say there are no plans for moving forces across the border into Ukraine's eastern and southern areas where the heaviest concentration of Russian-speakers live.
The Ukrainian Foreign Ministry was cautious at reports of a pull-back.
"We have information that the Russian Federation is carrying out unfathomable manoeuvres on the borders with Ukraine - in some border places they are taking away troops, in others they are coming closer," ministry spokesman Evhen Perebiynis said.
"Such action can not fail to cause concern especially since we today do not have a clear explanation from the Russian Federation about the aims of these movements," Perebiynis told journalists.
U.S. officials last Friday said Russia's reinforcement of troops near Ukraine had brought the total forces there to as many as 40,000, though European sources have been estimating lower numbers at around 30,000.
U.S. Secretary of State John Kerry said, after talks with Russian Foreign Ministry Sergei Lavrov on Sunday in Paris, that progress on resolving the crisis over the Russian annexation of Crimea depended on a Russian troop pull-back from Ukraine's borders.
Although Moscow says the buildup is part of military exercises, it has also been pressing diplomatically for Russian-speaking regions of Ukraine to be given greater autonomy and for the Russian language to be upgraded to the status of a state language in Ukraine. (Reporting by Natalia Zinets and Pavel Polityuk; Writing by Alessandra Prentice and Richard Balmforth; Editing by Alison Williams)