The BBC's Ian Pannell flees artillery fire at the ruins of Donetsk airportUkraine's government and separatist rebels have failed to begin withdrawing heavy weapons from the front line, despite a Monday deadline.
The two sides were given until two days after the latest truce came into effect to start the pullout.
Although most areas are quiet, fierce fighting is reported inside the town of Debaltseve, a key transport hub.
The leaders of Germany, Ukraine and Russia discussed the crisis in an overnight phone call.
Germany said they had agreed "concrete measures" for observers to have greater access, but gave no details.
Observers from the Organisation for Security and Co-operation in Europe (OSCE), who are charged with monitoring the ceasefire, have been trying to reach Debaltseve after being denied access by pro-Russian rebels on Sunday.
Ukraine's military said on Tuesday that five soldiers had been killed and 14 wounded in the past 24 hours. A spokesman told AFP news agency that most of the deaths happened near Debaltseve.
"The German chancellor and Ukrainian president called on the Russian president to exercise his influence on the separatists to enforce the ceasefire," German government spokesman Steffen Seibert said.
"Further, the withdrawal of heavy weapons should be started on Tuesday, as agreed in Minsk."
The ceasefire which came into effect on Sunday has been broadly observed, however separatists insist that the agreement does not apply in Debaltseve because they have the town surrounded.
Artillery strikes were reported overnight and on Tuesday morning around Debaltseve. Ukrainian military spokesman Anatoliy Stelmakh said separatists had continued to attack government positions.
Prominent Ukrainian journalist Andriy Tsapliyenko reported that pro-Russian forces had seized the railway station in Debaltseve.
A Ukrainian military spokesman quoted by Reuters confirmed that rebels were attacking the station but said it was still in government hands.
Rebels have offered Ukrainian troops under siege there a safe corridor to leave.
Senior rebel leader Denis Pushilin described Debaltseve as "internal territory" and said fighting for it was "a moral thing".
"We do not have the right (to stop fighting)," he told Reuters.
Another rebel leader, Andrei Purgin, said separatists planned to discuss the possible withdrawal of weapons later on Tuesday with representatives of Ukraine, Russia and the OSCE, Russian news agency RIA Novosti reported.
Under the ceasefire agreed in the Belarus capital Minsk last week, the withdrawal of heavy weapons from front line areas was due to start no later than the second day after the truce came into effect and be completed within two weeks.
The withdrawal would create buffer zones 50-140km (30-85 miles) wide.
But both sides says they will not withdraw heavy weapons until they see movement from the other side.