The West sounded more forceful warnings over Ukraine's battered truce Sunday as fighting raged around the port city of Mariupol and the warring sides wrangled over withdrawing heavy weapons.
The volatility was underlined by a bomb blast in the eastern Ukraine city of Kharkiv that killed two people during a pro-government march. Security officials said they had arrested four suspects linked to the "terrorist" attack.
Meanwhile, intensifying fighting around Mariupol, on the Avoz Sea, heightened concerns the pro-Russian rebels might be preparing a new offensive after storming the key town of Debaltseve in defiance of a week-old ceasefire.
Ukrainian defence spokesman Andriy Lysenko said "two tank attacks" were reported Sunday near Mariupol and fighting was ongoing near the city.
Kiev claims Russia has dispatched 20 tanks and other vehicles and big guns to the area.
"An advance on Mariupol would clearly be in breach of the agreements" underpinning the truce brokered by Berlin and Paris, German Foreign Minister Frank-Walter Steinmeier said in an interview with his country's Bild newspaper.
France's European affairs minister Harlem Desir, who was in Kiev for a unity march marking the first anniversary of the overthrow of Ukraine's former pro-Kremlin president, told reporters the ceasefire "must absolutely be respected".
Otherwise, he said, Western sanctions on Russia would be maintained "and could even be strengthened".
EU president Donald Tusk, also in Kiev, said he would "begin consultations on Monday to increase some of the measures in connection with the aggression" in Ukraine, the Interfax-Ukraine news agency reported.
On Saturday, US Secretary of State John Kerry went further, declaring that if the ceasefire continued to be violated, "there will be further consequences including consequences that will place added strains on Russia's already troubled economy".
Moscow denies the accusations it is behind the insurgency. Russia has already been hit by successive rounds of Western sanctions that are savaging its economy, which is headed for recession because of a collapse in oil prices.
Although the Ukraine ceasefire has managed to tamp down the combat along parts of the frontline, both sides in the conflict accuse the other of regular breaches.
On Sunday, the Ukrainian army and the rebels announced they had agreed to pull back heavy weapons from the frontline -- an important and overdue part of the truce.
But even that part of the deal turned contentious.
The insurgents said the withdrawal would only begin properly on Tuesday, after a couple of days of "preparation".
Kiev insisted that it was meant to start on Sunday with no delays, and be completed within two weeks.
About the only visible sign of compliance with the ceasefire agreement was a late-Saturday prisoner swap of nearly 200 fighters detained by both sides.
Several of the 139 soldiers traded by the rebels had been captured in the assault on Debaltseve, a strategic transport hub between the separatist strongholds of Donetsk and Lugansk.
The rebels overran the town on Tuesday, sending 2,500 Ukrainian soldiers fleeing.
A few of the released soldiers struggled on wooden crutches across a war-torn landscape cratered and littered with twisted metal.
One freed insurgent, Roman Biarinov, vowed he would quickly return to the fight. "We have to defend this land. We are going to win. I don't know how, but we will win," he said.
In Debaltseve on Sunday, the International Committee of the Red Cross distributed food, blankets and medical supplies to some 5,000 people who had been trapped in the town by the combat.
"We didn't have anything. No wages, no work, no potatoes," said one resident, Tatiana, who showed up to receive a box of supplies. "At least tonight we'll have something hot to eat."
Another, Albert Baronov, 76, raged at the fighting that destroyed his home and left him living for the past three and a half months in a cellar.
"We are furious that Europe let this happen," he told AFP. "Thousands of people have been killed and Europe hasn't stopped the war."