Monday morning saw some of the heaviest fighting in eastern Ukraine since the fall of Debaltsevo, almost a week after the signing of the second Minsk “cease-fire” agreement.

According to Ukrainian reports, hundreds of Russian-backed fighters took part in an assault, supported by tanks and artillery fire, on positions near the village of Starognatovka, in the south of the Donetsk region. The attack was repelled and Ukrainian forces made their first territorial gains since February 10. Since then, heavy artillery and Grad rockets have rained down across this section of the front line.

Kiev, which had already called a war cabinet on August 4, has responded by warning the Organization for Security and Co-operation in Europe (OSCE) and its Western partners that it reserves the right to return heavy weaponry, withdrawn in accordance with Minsk II, to the front line. Monday, Ukrainian President Petro Poroshenko ordered his foreign minister, Pavlo Klimkin, to arrange emergency consultations with the other members of the “Normandy Four” — France, Germany, and Russia.

While the Minsk ceasefire has been violated countless times since the fall of Debaltsevo, days after the signing of the agreement, this attack is of grave concern as it comes not only as the Kremlin and its proxies have raised the specter of “provocations” via threats of military action and attacks on the OSCE, but in an area of key strategic importance.

An OSCE statement released Tuesday said that recent attacks on their Special Monitoring Mission to Ukraine (SMM), including Sunday’s orchestrated protests and torching of four OSCE vehicles in separatist-held Donetsk and Lugansk, as well as two recent incidents in which SMM personnel came under fire, are endangering the role of the SMM in assisting with the implementation of Minsk II.

On the same day as the protests and arson attack on the OSCE, Evgeny Buzhinsky, a Moscow think tank head and former member of Russia’s General Staff, told the BBC that any Ukrainian move over the Minsk demarcation line could result in a full-scale military response from Russia.

Buzhinsky has made extreme threats before, claiming in February this year that any delivery of lethal weaponry to Ukraine by the USA would be seen by the Kremlin as a “declaration of war.”

He has also been keen to spread the claim that any Western intervention in the Ukrainian conflict could trigger nuclear war, a remote possibility that has been taken in by some of the more hysterically inclined members of the Western media.

The point of such statements is usually to scare off Westerners, to make them fear that any opposition to Russia’s plans may result in a nuclear holocaust. However, such high-profile individuals (and note here that Buzhinsky only retired from the General Staff in 2009) are likely used to disseminate specific political messages from the Kremlin, and the timing of Monday’s statement could well be a threat ahead of renewed military provocations in the Donbas.

And, in response to the attack Monday morning, judging from front-line reports, Ukraine did indeed go over the Minsk demarcation line as it launched a counter-attack.

In the early hours of August 10 Russian-backed forces pounded Ukrainian positions near the villages of Nikolaevka, Novogrigoryevka, Bogdanovka, and Starognatovka with artillery and heavy mortars. Then, around 3:20 a.m., about 400 fighters launched an assault on a defensive position held by the Ukrainian 72nd Mechanized Brigade, near Starognatovka. They were supported, the Ukrainian Defense Ministry said, by 10 tanks and 10 infantry fighting vehicles.

Aleksandr Samarsky, deputy commander of the 72nd , told a Ukrainian reporter: “Right now we have a contact fight, practically hand-to-hand. At first they hit our blockpost, apparently to zero in. They then shifted their fire on our positions and the village. There is destruction. We returned fire, after which their infantry went up to our positions.”

An hour later, the 72nd repelled the attack and were now on the offensive, pursuing the Russian-backed fighters back toward their own positions.

In fact, by 6 a.m., Ukrainian troops had entered the separatist-held village of Novolaspa, around 8 kilometers east of Starognatovka, and were pushing out the Russian-backed fighters street by street. But then, having taken control of Novolaspa, Ukrainian troops were ordered to withdraw for fear of being seen as in violation of Minsk II.

Perhaps heeding warnings such as that issued via Buzhinsky, Vladislav Seleznyov, a spokesman for the Ukrainian General Staff, has denied that Ukrainian troops ever entered Novolaspa, stressing that the village lay on the separatist side of the Minsk demarcation line and Ukraine had not violated the agreement.While Ukrainian troops were pulled back from Novolaspa, Kiev has made net territorial gains, taking control of a strategic hilltop just outside the village, allowing them commanding views over the valley of the river Kalmius, along which lie separatist-held towns and military positions. This is the first time that Ukrainian forces have gained ground since their brief offensive on February 10.

The Russian-backed separatists have not released any casualty figures for Monday’s battle, but Misyura claimed “dozens if not hundreds” dead. The fighting was also costly for the Ukrainians: While the official report says one soldier was killed and nine wounded, Yana Zinkevych, chief of Pravyi Sektor’s medical unit, reported that seven Ukrainian fighters, three of them Pravyi Sektor members, had been killed and 11 wounded. (Pravyi Sektor is a highly controversial, far-right volunteer battalion fighting on behalf of Kiev.) Later Monday night, a volunteer group claimed that 19 wounded soldiers had been brought into a medical centre in Volnovakha over the last 24 hours.

Since the end of the battle, Ukrainian-held positions along this stretch of front have been subjected to repeated bombardment with heavy artillery and Grad rockets. These attacks continued throughout Monday, Monday night and into Tuesday. There are also reports that Ukraine has responded by shelling separatist-held settlements, most notably Telmanovo, where photos uploaded Monday showed smoke rising after alleged Grad bombardment by Ukrainian forces. The separatist-backed head of the Telmanovo administration, Sergei Ivanov, claimed Monday that a civilian woman had been killed.

Should these embers lead to all-out conflagration in this area of the front line, the implications for Mariupol, a city of enormous economic importance to the Donetsk region, would be grim. Mariupol, which was taken over by Russian-backed separatists for over a month last year before being retaken after two hard-fought battles, is of not only symbolic but vital economic significance to the region, as both a port and vast centre for steel production. It’s also a necessary win for Moscow’s proxies if Russia is to ever establish a “land bridge” from its own territory to illegally occupied and annexed Crimea.

What makes the latest flare-out worrisome is that it’s occurred in precisely the swathe of Ukrainian-held territory that any Russian pincer move on Mariupol would have to come.

To the west of Starognatovka, across sparsely inhabited fields, lies the Donetsk-Mariupol highway. Ukrainian troops hold most of this road, from Mariupol on the Azov coast up to separatist-held Yelenovka, south of Donetsk. Russia needs to push Ukrainian forces back from the river Kalmius and off the highway.

And even before Monday’s massive assault, it was clear that Russia was militarizing separatist-held territory along the eastern banks of the Kalmius.

For several months now, the OSCE has reported spotting concentrations of armor in several towns in this area: Komsomolskoye, Razdolnoye, Michurino, and Telmanovo. At the end of June, the Ukrainian Dnipro-1 Battalion released drone footage revealing the rapid construction of a military base in the woods south of Sontsevo, complete with tanks, fuel bowsers and communications equipment. The speed and professionalism of the construction clearly indicates Russian military engineers’ work. The OSCE has also reported encountering military personnel in Russian uniforms in Petrovskoye, only a short distance north of Novolaspa. Our team at The Interpreter has documented, using abundant video and photographic evidence, the existence of a military training facility on the grounds of an agricultural college in Razdolnoye.

Ukraine is clearly aware of the importance of this front, and may well have already taken action to hinder the supply of armor from Russia: On July 6, a road bridge over the Kalmius in Razdolnoye was severely damaged by a blast, making the highway impassible to heavy vehicles. Kiev has not commented on the incident but the separatist claim that Ukrainian saboteurs planted explosives on the bridge.

Even if it takes a while for the bridge to be repaired, Russia can still send armor to territory north of Razdolnoye via the Uspenka border crossing, and to the south via occupied Novoazovsk, on the coast.

Russia may not be about to assault Mariupol right now, but the months of military buildup, followed by Monday’s assault, not to speak of the near-daily shelling along this section of the front line, indicates that such an offensive is a serious possibility.