Hopes that Thursday's deal will end a conflict that has killed more than 5,000 people have been dampened by the collapse of an earlier truce last month.
The European Union kept pressure on Russia and the rebels by announcing a new list of separatists and Russians targeted with sanctions on Monday. Moscow promised an "adequate" response.
The United States said it was "gravely concerned" by the fighting in and around Debaltseve and that it was closely monitoring reports of a new column of Russian military equipment moving toward the region.
"These aggressive actions and statements by the Russia-backed separatists threaten the most recent ceasefire," U.S. State Department spokeswoman Jen Psaki said in a statement.
"We call on Russia and the separatists it backs to halt all attacks immediately."
Russia denies sending arms or troops to back the rebels in mainly Russian-speaking areas of east Ukraine despite what Kiev and the West say is incontrovertible proof.
The separatists said soon after the ceasefire came into effect they had no intention of observing it at Debaltseve, where they have been advancing since January and now have a Ukrainian unit all but encircled.
The OSCE said on Sunday the rebels had refused to allow its monitors into Debaltseve after the truce took effect.
Fighting began in east Ukraine after the overthrow of a Moscow-backed president in Kiev a year ago and Russia's annexation of the Crimea peninsula a month later.
The West fears Putin, who has called parts of Ukraine "New Russia," wants the conflict to fester for years so that Kiev cannot control east Ukraine and is prevented from joining NATO, while Russia can retain influence there.
Moscow accuses the West of waging a proxy war in Ukraine to seek "regime change" in Moscow and "contain" Russia.
Western countries say they reserve the option of expanding economic sanctions on Moscow over the crisis, hoping a growing financial crisis in Russia will persuade Putin to use his influence with the rebels to stop the fighting.
Some analysts say Putin is banking on Ukraine's economy collapsing much faster than Russia's and on Russians remaining united behind him over the seizure of Crimea, which has boosted his popularity ratings at home.