Ukrainian troops have launched a massive assault on militia-held areas Sunday morning after an order from Kiev, a presidential aide said. The self-proclaimed Donetsk republic’s leader accused Kiev of trying to restart the war.

The order to launch the offensive was issued early approximately at 6:00 am, according to Yury Biryukov, an aide to President Petro Poroshenko.

“Today we will show HOW good we are at jabbing in the teeth,” he wrote on his Facebook page, a mode of conveying information favored by many Ukrainian officials.

In a later post he said: “They are now striking a dot. Uuu…” in a reference to Tochka-U (‘tochka’ means ‘dot’ in Russian), a tactical ballistic missile, one of the most powerful weapons Ukraine so far deployed against rebel forces.

“That wasn’t a dot but ellipsis. Strong booms,” he added.

Reports from the ground confirmed a sharp escalation of clashes across the front line, with particularly heavy artillery fire reported at Gorlovka.

“Locals in Donetsk said they haven’t heard such intensive shelling since summer,” Valentin Motuzenko, a military official in the self-proclaimed Donetsk People’s Republic, told Interfax news agency.

“The Ukrainian military are using all kinds of weapons, Grad multiple rocket launchers, mortars…” Motuzenko said.

A source at the site told RT that a group of OSCE monitors have demanded that the Ukrainian military stop the shelling. The source added that there is a danger that the group may itself come under artillery fire.

READ MORE: OSCE identifies direction of deadly Donbass bus rocket

Aleksandr Zakharchenko, who was elected in November the prime minister of the self-proclaimed Donetsk People’s Republic, has accused Kiev of using the ceasefire to regroup their troops and boost their armaments, and now of trying to restart the all-out war.

“It’s clear that Kiev is making an attempt to take revenge for the serious military defeat it took from the militia last year,” he added. “Kiev always ignored our numerous suggestions to pull back heavy weapons from the disengagement line.”

“At the moment, we are aware of five civilians killed,” the Donetsk administration said, adding that the shelling of the city by Ukrainian forces is still under way.

Several residential buildings, a shop and a bus station have been seriously damaged by artillery fire in the city, RIA Novosti reported.

The Putilovsky Bridge, which is located near the Donetsk airport, has also been destroyed in the shelling, RT’s Roman Kosarev reported from the scene.

There were also reports of attacks on the town of Makeevka and several nearby villages.

The militia added that at least one shell hit a residential area in central Donetsk rather than the outskirts of the city.

Two people have died and 16 other received injureies after a Ukrainian shell exploded at bus station in city of Gorlovka, the press service of the People’s Republic of Donetsk said.

Several buildings in the city also suffered direct hits during the bombardment, including four schools and a kindergarten, the press service added.

Witnesses said that Kiev used aviation in the assault, with Sukhoi Su-24 warplanes jets seen in the air above Gorlovka.

The ministry of Foreign Affairs of the People’s Republic of Donetsk (DPR) has called upon the international community to address the worsening of the situation in southeast Ukraine and “make every effort to stop unpunished mass murder in the heart of Europe."

“The People’s Republic of Donetsk again suffered a treacherous and large-scale attack from Ukraine,” which is aimed at the destruction of the republic’s population and infrastructure, Aleksandr Kofman, DPR foreign minister, said in a statement.

The actions of the Kiev forces in Donetsk Region “bear all characteristics of serious war crimes,” Kofman said.

Moscow is seriously concerned by the by the renewal of shelling of residential areas of the eastern Ukrainian city of Donetsk by Kiev forces, the Russian president’s press secretary, Dmitry Peskov, said.

The actions of the Ukrainian military in no way promote the settlement of the conflict between Kiev and the eastern regions, he said.

"Of course, the stance of the Russian side will be expressed. Of course, it involves serious concerns over the resumption of full scale warfare in Donetsk, including shelling of populated residential areas. And, of course, such a state of affairs doesn’t promote the fulfillment of the Minsk agreements [from September 5] and the further search for a peaceful solution,” Peskov told Govorit Moskva radio station.

The escalation comes after several days of violent clashes at the ruins of the Donetsk International Airport, a scene of constant fighting over the months of shaky ceasefire between Kiev’s troops and local militia forces.

The militia, who have accused the Ukrainian military of launching constant attacks on Donetsk from their positions at the airport, reported taking over the location this week. Kiev denied this report.

The Ukrainian Security and Defense Council confirmed the resumption of hostilities and that Kiev's objective was to recapture the airport.

Kiev launched it's retaliation days after President Poroshenko ordered a new wave of mobilization into the country’s armed forces. The government plans to hold at least three conscription campaigns over 2015 to sustain its military operations in the east.

The escalation of violence comes after a controversial incident at Kiev’s checkpoint near the town of Volnovakha, where 12 passengers were killed on Tuesday. Kiev blamed the deaths on the militia, saying it was hit by a Grad rocket. The DNR forces denied the accusations and said the bus was targeted by the Ukrainian troops themselves – either in a tragic incident or as a staged provocation – and said the damage visible in the picture of the bus is consistent with a less powerful weapon, possibly a landmine.

An OSCE observer team studied blast craters near the checkpoint and confirmed the direction they came from. Russia’s permanent representative to the OSCE, Andrey Kelin, said the direction runs along the disengagement line separating militia- and Kiev-controlled areas, which makes it difficult to determine who fired the rockets. The observers did not identify the type of rockets used.