US President Barack Obama has urged Russia to "move back its troops" on Ukraine's border and lower tensions.

Russia is believed to have massed a force of several thousand troops close to Ukraine's eastern frontier. According to some estimates there are around 30 000 troops, while some US generals believe the number can be as high as 80 000. 

Russia claims the troops are there for military exercises. 

"We've seen no specific indications that these -- that exercises -- are taking place," Pentagon spokesman Rear Adm. John Kirby told reporters, quoted by Fox News. 

Meanwhile, The Wall Street Journal reported on Friday that Russian troops are concealing their positions and establishing supply lines, stoking fears that they are actively preparing for invasion in eastern Ukraine. 

Meanwhile, a Russian security official has said intelligence measures are being stepped up to counter Western threats to Moscow's influence.

"There has been a sharp increase in external threats to the state. The lawful desire of the peoples of Crimea and eastern Ukrainian regions is causing hysteria in the United States and its allies," Federal Security Service (FSB) deputy head Alexander Malevany was quoted as saying by Interfax.

He said Moscow was taking "offensive intelligence measures" to counter Western efforts to "weaken Russian influence in a region that is of vital importance".

NATO Secretary-General Anders Fogh Rasmussen said the organisation was extremely worried about a "huge military build-up" on Ukraine's borders.

He said NATO had plans in place to ensure effective defence and protection for all its members, including the three Baltic states - Estonia, Latvia and Lithuania.

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