The Organization for Security and Co-operation in Europe currently monitors two checkpoints on the volatile border of eastern Ukraine and Russia, which make up just one kilometre of a frontier hundreds of kilometres long, the US mission to the OSCE said in a statement.
"Regretfully, this continued obstruction is yet another missed opportunity for Russia to match words with action and to contribute to de-escalation," said Ambassador Daniel Baer.
"There should be no illusions — the observer mission as it stands is inadequate."
The US representative to the European security body also said Russia has rejected "a modest increase in the number of observers" to reduce the "excessive workload" of the 16-member team currently in place on the border.
Baer also called on Russia to fully implement the peace deal reached last month in the Belarussian capital Minsk between Kiev, Moscow and the pro-Russian separatists in east Ukraine. The accord includes OSCE monitoring and verification on both sides of the border.
"Russia must also withdraw its fighters and heavy weapons from Ukraine and return all hostages, as Russia signed up to do at Minsk," he added.
Moscow denies involvement in the east Ukraine uprising and rejects charges that it is trying to seize its neighbour's most economically important industrial region in retaliation for the February ouster of a Russian-backed president.