United States Secretary of State John Kerry accused pro-Russia rebels of taking a "piecemeal approach" to the ceasefire agreement in east Ukraine.
While speaking at the United Nations, Kerry said that the separatists had only withdrawn heavy weapons from selective areas along the frontline, reported the BBC.
The statement came as Russian Foreign Minister Sergei Lavrov claimed that there had been "tangible progress" with the truce.
Both Ukraine and the rebels have said they are withdrawing heavy weapons from the frontline in keeping with the truce deal struck in Minsk last month.
Monitors from the OSCE security group have reported weapons movements on both sides but said that it was too early to confirm a full withdrawal.