The cease-fire in Ukraine that came into effect during the night is generally holding, despite some reports of scattered shelling.
The main point of concern on February 15 was the area around the town of Debaltseve, where there has been heavy fighting in recent days. A commander for the pro-Russian separatists in the area told the Reuters news agency that his side does not believe the cease-fire applies to Debaltseve because "it is our territory."
A Ukrainian government spokesman said Kyiv's forces had come under fire about 10 times since the midnight cease-fire but that all shelling was "localized" and no service personnel had been killed.
A Ukrainian military spokesman said Kyiv has prepared storage areas for heavy weaponry that is due to be removed from the line of conflict between the government and Russia-backed separatists in eastern Ukraine.
Ukrainian President Petro Poroshenko ordered Ukrainian military forces to halt fire in the east of the country at midnight on February 15 in line with the new cease-fire agreement.
Poroshenko said the "world was holding its breath," hoping the cease-fire would hold.
But the Ukrainian leader added that intense shelling by pro-Russian rebels on Debaltseve was threatening peace efforts.
Pososhenko warned that Ukraine, if slapped once, would not offer the other cheek.
Under an agreement struck last week, both Ukrainian forces and Russian-backed separatist rebels were to hold their fire as of one minute after midnight, local time, on February 15.
A halt in shelling was reported in the rebel-held city of Donetsk after midnight.
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.
However, within hours both sides were trading accusation of truce violations.
Ukrainian security services chief Valentyn Nalyvaichenko said one infringement was reported about 50 minutes after the deadline.
Artillery salvoes were fired from an area that Nalyvaichenko said is under the control of a Cossack unit manned by Russian citizens.
The rebels accused Ukrainian forces of deploying artillery shortly after midnight.
Donetsk News Agency, a separatist mouthpiece, quoted rebel leader Eduard Basurin as saying the Ukrainian forces garrisoned in Debaltseve fired artillery and mortars at rebel positions.
Earlier, U.S. officials expressed concern about rebel shelling of Debaltseve, a government-held town midway between the two largest rebel strongholds of Luhansk and Donetsk.
U.S. Secretary of State John Kerry urged implementation of the cease-fire in a telephone call with his Russian counterpart Sergei Lavrov and expressed concern about efforts by Russia and the separatists to cut off Debaltseve.
U.S. President Barack Obama also expressed "deep concern" about the violence around Debaltseve in a telephone call with Poroshenko, the White House said.
Obama also talked to German Chancellor Angela Merkel, who negotiated the cease-fire 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 cease-fire and protocol agreements reached at Minsk last September and reaffirmed by the Minsk Implementation Plan this week," the White House said.
The U.S. State Department on February 14 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.
Russia has repeatedly denied repeated Western claims that it has sent troops and equipment to the rebels.
Ahead of the cease-fire deadline, statements by rebel leaders raised questions about whether they would respect the truce.
Alexander Zakharchenko, the rebel leader in the city of Donetsk, was quoted by the RIA Novosti news agency as saying his fighters would not allow Ukrainian forces to escape Debaltseve.
Separatists have said the Ukrainian troops there would be offered only the opportunity to surrender.
The Reuters news agency said its reporters witnessed a column of new military vehicles and artillery passing through the checkpoint in the direction of Debaltseve on February 14.
Reuters said the checkpoint was manned by several dozen professional-looking combatants. Tanks and armored 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."
With reporting by Reuters and AP