Shelling stopped abruptly at midnight on Saturday in the eastern Ukrainian city of Donetsk after President Petro Poroshenko ordered government forces to halt firing in line with a ceasefire agreement aimed at ending the country's bloody conflict.
Poroshenko, wearing the uniform of the armed forces supreme commander, said in a televised address in the capital Kiev that there was still "alarm" over the situation around Debaltseve, a key transport hub, where government forces have been hard pressed by encircling Russian-backed separatists.
The ceasefire, negotiated in four-power talks on Thursday, foresees creation of a neutral "buffer zone" and withdrawal of heavy weapons responsible for many of the 5,000 deaths in a conflict that has caused the worst crisis in Russia-West relations since the Cold War a generation ago.
Kiev and NATO have long charged that Moscow has supplied pro-Russian separatists in eastern Ukraine with arms and men, and Washington and its allies have slapped Russia with a series of economic sanctions. Russian President Vladimir Putin denies Moscow is involved in fighting for territory Putin calls "New Russia," although Western officials cite overwhelming evidence to the contrary.
Pososhenko warned on Saturday night that Ukraine, if it was slapped once, would not offer the other cheek. But, seated alongside armed forces chief of staff Viktor Muzhenko, he added: "I very much hope that the last chance to begin the long and difficult peaceful process for a political settlement will not be wasted."
"As a first step I now give the order to the armed forces of Ukraine... to cease fire at 00:00 hours on February 15," he said.
Military spokesman Vladyslav Selezynov said the Ukrainian armed forces immediately fulfilled Poroshenko's order and the big guns fell silent in Donetsk and some other parts of the separatist-leaning east.
In the hours leading up to the ceasefire, heavy artillery and rocket fire roughly every five seconds had reverberated across Donetsk, the main regional city in the east which is under the control of the secessionists.
In Artemivsk, a town in government-controlled territory north of Debaltseve that has been hit twice in two days by rocket attacks, there was also silence at midnight.
US Secretary of State John Kerry urged implementation of the ceasefire in a telephone call with his Russian counterpart Sergei Lavrov and expressed concern about efforts by Russia and the separatists to cut off Debaltseve, the US State Department said.
US President Barack Obama also expressed "deep concern" about the violence around Debaltseve in a telephone call with Poroshenko, the White House said.
A member of a Ukrainian pro-government unit near the eastern town of Horlivka, who only gave his nickname of Turnir, said on television channel 112 soon after midnight: "It's quiet. It's been quiet for half an hour. But we are waiting. We don't believe them. For the past three days they have been banging us hard."
Debaltseve, a strategic rail junction that lies in a pocket between the two main rebel-held regions, has been the focus of some of the fiercest recent fighting.
Four shells hit Donetsk on Saturday, blasting craters in the streets. A Reuters witness saw one dead body from the attacks.
Obama also talked to German Chancellor Angela Merkel, who negotiated the ceasefire this week with the leaders of Ukraine, Russia and France in Minsk, the capital of Belarus.
Obama and Merkel "agreed on the pressing need for all signatories to implement the ceasefire and protocol agreements reached at Minsk last September and reaffirmed by the Minsk Implementation Plan this week," the White House said.
The 15-member United Nations Security Council was discussing a Russian-drafted resolution that would welcome the ceasefire agreement and call on all parties to fully implement it. Diplomats said the council could vote as early as Sunday on the draft resolution.
Weapons around Debaltseve
The US State Department, pressing its case that Russia was backing the rebels in the latest fighting, on Saturday released three commercial photographs that spokeswoman Jen Psaki said showed "the Russian military has deployed large amounts of artillery and multiple rocket launchers around Debaltseve, where it is shelling Ukrainian positions."
"We are confident that these are Russian military, not separatist systems," she said.
On Saturday, Ukrainian military spokesman Andriy Lysenko said at a televised briefing in Kiev: "Ahead of midnight, rebels are trying to complete tactically important plans to enlarge the territory under their control, primarily in the direction of Debaltseve."
Heavy shelling could be heard at a rebel checkpoint about 15 km (10 miles) west of Debaltseve, a Reuters witness said, reporting outgoing artillery rounds almost every minute.
A column of new military vehicles and artillery passed through the checkpoint in the direction of Debaltseve. The checkpoint was manned by several dozen professional-looking combatants. Tanks and armoured vehicles could also be seen.
Speaking on condition of anonymity, a rebel at the checkpoint said local fighters were being supported by "guests from Russia."
Spokesman Lysenko said separatist forces continued to be reinforced by fighters and military equipment crossing Ukraine's eastern border from Russia over the past 24 hours.
The rebels have advanced far past the line of an earlier ceasefire deal, agreed in September. The new accord appears to envisage them withdrawing their guns around 75 km (50 miles), to take them back behind it, while Ukrainian guns would move 25 km (15 miles) back.
Thursday's accord also prescribed constitutional reform to give more autonomy to eastern Ukraine, where many Russian speakers live. Kiev rejects independence for the "people's republics" the rebels have declared.