Foreign Secretary David Miliband is seeking to build an international consensus to pressure Russia into leaving Georgia completely.
Columns of Russian armour moved out of territory troops were holding deep inside the former Soviet republic on Friday, which Mr Miliband welcomed as a "step forward".
But the West accused Moscow of failing to live up to the commitment in an EU-brokered truce to return to the positions held before violence erupted in the separatist Georgian province of South Ossetia.
There has been condemnation of the decision to maintain a military presence in a so-called security zone on the fringes of South Ossetia's border.
Before the conflict began earlier this month Russia had peacekeepers within the province, but as fighting progressed troops moved further into Georgian territory.
Mr Miliband said: "I remain deeply concerned that Russian forces have not withdrawn to the pre-August 7 position as agreed.
"It is imperative that Russian forces fully and speedily implement and abide by the commitments that it has made.
"We will consult widely to build consensus behind this position and ensure that law not force defines the conclusion of this crisis."
Mr Miliband's views were shared by the US and France, which currently holds the EU presidency.
US president George Bush discussed the withdrawal with French President Nicolas Sarkozy and they "agreed that Russia is not in compliance and that Russia needs to come into compliance now", the White House said.