Ukraine crisis: Army and rebels stall heavy arms pullout
The two sides were given until two days after the latest truce came into effect to start the pullout.
The government said it would not pull back until fighting ended in the beleaguered town of Debaltseve.
The leaders of Germany, Ukraine and Russia discussed the crisis in an overnight phone call.
Germany said on Tuesday they had agreed "concrete steps" 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 the rebels on Sunday.
"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 broadly been observed, however separatists insist that the agreement does not apply in Debaltseve because they have the town surrounded.
They have offered Ukrainian troops encircled there a safe corridor to leave.The Ukrainian military command said pro-Russian rebels had fired on government positions 112 times on Sunday, mostly around Debaltseve.
A Ukrainian officer said there was also fighting near the southern port city of Mariupol. The rebels accused Ukrainian forces of shelling Donetsk airport.
'Not an indicator'
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 a Ukrainian military spokesman quoted by Reuters news agency said there would be no withdrawal of heavy weapons yet because the rebels were violating the ceasefire.
"The pre-condition for withdrawal of heavy weapons is fulfilling Point One of the Minsk agreements - the ceasefire. One hundred and twelve attacks are not an indicator of a ceasefire," said Andriy Lysenko.
Rebel commanders also said there were no grounds yet to withdraw heavy weapons from the combat zone.
Rebel "defence minister" Vladimir Kononov said: "We will withdraw heavy weapons from the contact line if we get a definite sign that the Ukrainian side has started doing the same thing."
'Fragile situation'
German Chancellor Angela Merkel and French President Francois Hollande have previously called for the OSCE to be granted free access to eastern Ukraine."The situation is fragile," Mrs Merkel said. "It was always clear that much remains to be done. And I have always said that there are no guarantees that what we are trying to do succeeds. It will be an extremely difficult path."
Meanwhile, US state department spokeswoman Jen Psaki said the US was "gravely concerned by the deteriorating situation in and around Debaltseve" and called on "Russia and the separatists it backs to halt all attacks immediately".
Reuters reporters near Debaltseve said that it was being ceaselessly bombarded with artillery.
They said that at least six tanks as well as armoured personnel carriers and artillery were present in woods 10km (six miles) west of Debaltseve near Vuhlehirsk, which the rebels seized a week ago.
Meanwhile, further EU sanctions against Russia have gone into effect.
The new sanctions list targets 19 officials - most of them in the pro-Russian separatist strongholds of Donetsk and Luhansk, but also two Russian deputy defence ministers and a Russian celebrity singer and MP, Iosif Kobzon.
Officials say more than 5,400 people have been killed since the conflict erupted in eastern Ukraine in April, but the UN believes the actual death toll to be much higher.