WASHINGTON—Russian troops massing near Ukraine are actively concealing their positions and establishing supply lines that could be used in a prolonged deployment, ratcheting up concerns that Moscow is preparing for another major incursion and not conducting exercises as it claims, U.S. officials said.

Such an incursion could take place without warning because Russia has already deployed the array of military forces needed for such an operation, say officials briefed on the latest U.S. intelligence. (Follow the latest developments...