The reports of fresh shelling came just hours after the Ukrainian military claimed that a serviceman had been killed and several others wounded during clashes with Moscow-backed rebels near the city of Mariupol.
“On 26 April, the SMM [Special Monitoring Mission] observed what it assessed to be the most intense shelling in Shirokino since fighting began in the area in mid-February 2015,” observers from the Organization for Security and Co-operation in Europe (OSCE) reportedly said.
Shirokino is located about 14 miles east of Mariupol -- a government-held coastal city in southeastern Ukraine.
According to the OSCE monitors, at least 11 tanks were observed in the region, which has been under the control of Ukrainian forces since February.
Earlier on Sunday, Ukrainian military had alleged that separatists had stepped up attacks against government forces, putting the fate of a ceasefire deal -- signed in Minsk, Belarus, in February -- at risk.
A Ukrainian army spokesman earlier said that the rebels were using large-caliber artillery in violation of the Minsk accord, which calls for heavy weaponry to be pulled back from the conflict zone.
“An increase in the number of violations of the ceasefire regime by [separatist] fighters has been noticeable in the past 24 hours,” Ukrainian military spokesman Oleksander Motuzyanyk reportedly said.
Over 6,100 people, both civilians and military personnel, have been killed in eastern Ukraine since the start of the conflict, which began after Russia annexed the Black Sea peninsula of Crimea in March last year.
While most world leaders condemned the annexation, calling it a “violation of Ukraine’s sovereignty and territorial integrity,” Russian President Vladimir Putin has consistently defended the move.
Most recently, in an interview broadcast Sunday, marking Putin’s 15 years in power, Putin said he did not regret his decision.
“I believe that we did the right thing, and I do not regret anything,” Russia’s Sputnik News quoted Putin as saying about Crimea’s “reunification” with Russia.
“I am deeply convinced that we do not violate any rules of the game. … This refers to our relations with Ukraine, it refers to the situation in Crimea, it refers to our efforts with regard to the fight against international terrorism in other regions of the world.”