WASHINGTON, Feb 6 (AFP) - Russia has stepped up preparations for an all-out invasion of Ukraine but it is not clear if Moscow has decided to take such a step, US officials have said citing intelligence assessments.

Russia has assembled 110,000 troops along the border with its pro-Western neighbour but US intelligence has not determined if President Vladimir Putin has actually decided to invade, according to the officials who in recent days briefed members of Congress and European allies.

The officials warned lawmakers that the assembled Russian force on the frontier is growing at a rate that would give Putin the force he needs for a full-scale invasion -- some 150,000 soldiers -- by mid-February.

They said Putin wants all possible options at his disposal: from a limited campaign in the pro-Russian Donbas region of Ukraine to a full-scale invasion.

Russia denies that it is planning to invade Ukraine.

If Moscow does opt for a full-scale attack, the invading force could take the capital Kyiv and topple President Volodymyr Zelensky in a matter of 48 hours, the officials said.

They estimated such an attack would leave 25,000 to 50,000 civilians dead, along with 5,000 to 25,000 Ukrainian soldiers and 3,000 to 10,000 Russian ones.

It could also trigger a refugee flood of one to five million people, mainly into Poland, the officials added.

President Joe Biden has decided to send American forces to Poland to protect NATO members, as diplomats work furiously to try to persuade Russia to pull its troops back from the border with Ukraine.