OSCE observers said on Monday they have witnessed the most intense shelling in the flashpoint eastern village of Shyrokyne since fighting began there in mid-February, as well as movement of heavy weapons.
The Vienna-based Organization for Security and Cooperation in Europe, which is helping to monitor a ceasefire in Ukraine, said that explosions 300 metres from one of its observation posts forced its personnel to change location.
"On 26 April, the SMM (special monitoring mission) observed what it assessed to be the most intense shelling in Shyrokyne... since fighting began in the area in mid-February 2015," the OSCE said in a report issued overnight.
"For the last 12 hours, and on-going at the time of this report, the SMM has observed sporadic to continuous exchanges of fire involving small arms, machine gun, rocket propelled grenade and automatic grenade launcher," the security body said.
Shyrokyne, near the strategic city of Mariupol, has been the scene of frequent violations of an internationally brokered February ceasefire deal between Ukrainian government forces and pro-Russian separatists.
Throughout Sunday, from two adjacent observation posts east and west of Shyrokyne, the OSCE observed 69 outgoing tank shells, 191 outgoing 82-millimeter mortar rounds and 153 outgoing 120mm mortar rounds.
On Sunday morning two "loud" explosions, most likely artillery, went off some 300 metres (yards) from an OSCE observation post in a government-held village 1.5 kilometres (0.9 miles) west of Shyrokyne, forcing the observers to move to another post, it said.
In the early afternoon, the shelling was heard all the way to Mariupol, held by the Ukrainian government 20 kilometres away.
An OSCE drone observed 11 tanks and four armoured personnel carriers (APCs) on Sunday, with infantry mounted, moving through rebel-held Kulykove, 15km north of Shyrokyne, the OSCE said.
Additionally, over the previous three days, the drone sighted 17 tanks, three self-propelled howitzers and 60 APCs in a rebel-controlled area 50 kilometres north of Shyrokyne, the OSCE said.
Under the ceasefire, heavy weapons are meant to have been withdrawn from frontlines.
The conflict in eastern Ukraine has killed more than 6,000 people in the past year and displaced more than a million, according to the United Nations.