The international community must not lower its guard and should keep "everyone's feet to the fire" after a fresh ceasefire deal in Ukraine, the UK deputy prime minister advised.
Nick Clegg welcome the deal after Russian President Vladimir Putin announced a ceasefire would take effect from midnight on 15 February.
The agreement comes after almost 16 hours of talks in Minsk between the leaders of Ukraine, Russia, France and Germany.
But the Liberal Democrat leader urged Western leaders to take a cautious approach after Putin's announcement, warning that "we have been here before".
"Any agreement, any statement from those leaders... that they are going to enter into a ceasefire is, of course, welcome," he told LBC Radio.
"All I would say is that we have been here before. It is incredibly important that we keep everyone's feet to the fire, we don't lower our guard."
The conflict in Ukraine between pro-Russian separatists and Kiev forces has killed at least 5,300 people so far.
A key point in the talks was who controlled the 400km stretch of Russia's border with rebel-controlled Ukraine.
The parties have reportedly not been able to reach an agreement on this demarcation line.
Separatist have gained ground in recent weeks, pushing government troops back from positions agreed in a September ceasefire deal.
Alexander Zakharchenko and Igor Plotnitsky, separatist leaders, had joined the contact group, which includes representatives from Ukraine, Russia and the Organization for Security and Co-operation in Europe (OSCE), but critics argued they were directly controlled by the Kremlin.
The Ukraine and the West have accused Putin of aiding the rebels with troops and weapons, but this has always been denied by the Kremlin.