Fighting is continuing in eastern Ukraine more than a day after a ceasefire was supposed to take effect.
The Ukrainian military command said the pro-Russian rebels had attacked 112 times since early Sunday, mostly in the bitterly contested Debaltseve area.
A Ukrainian officer said there was also fighting near Mariupol, a port city.
The rebels accused Ukrainian forces of shelling Donetsk airport. 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.
The Russian foreign ministry said the latest sanctions, which include asset freezes and travel bans across the EU, "look especially ridiculous against the background of the Minsk [ceasefire] accords."
But an EU official said the sanctions were intended to punish the Russian-backed rebels for a rocket attack on Mariupol last month which killed more than 30 civilians.
"The sanctions were meant to come into force last week, but they were postponed because the EU didn't want to interfere with the Minsk talks. But they always intended to implement these sanctions, and today, they have," the official said.
Observers from the OSCE security body are to try again to reach the besieged Ukrainian town of Debaltseve. They said they were denied access by the rebels on Sunday.
German Chancellor Angela Merkel and French President Francois Hollande have called for the OSCE to be granted free access to eastern Ukraine.
Along with Ukrainian President Petro Poroshenko, they also expressed their concern over the ongoing fighting.
In the past 24 hours, five Ukrainian soldiers died in fighting at Shirokyne, near Mariupol, the Ukrainian military said.
The rebels say they have Debaltseve encircled so it should be considered theirs.
On Monday, rebel commanders 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".
On the Ukrainian side, a military spokesman quoted by Reuters news agency said there would be no withdrawal of heavy weapons yet because the rebels were violating the ceasefire.
Elsewhere on the front line reports say the ceasefire has broadly been observed.
But according to the Minsk agreement, reached last week, the ceasefire is only the first step.
It should be followed on Monday by a withdrawal of heavy weapons - lasting two weeks - to create buffer zones 50-140km (30-85 miles) wide.
Analysts point out that previous ceasefires initially appeared to be holding but eventually failed, and say the next 48 hours are critical.
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.
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