Kiev/Moscow, Aug 5 : Fierce clashes took place Ukrainian and Russian forces as the two warring sides fought for control of Avdiivka and Bakhmut in the contested eastern region of Donetsk. Troops loyal to Moscow are trying to drive the Ukrainian military out of their positions, dpa news agency quoted reports as saying from both countries. The city of Donetsk itself has been controlled by pro-Russian separatists since 2014, but the Ukrainian army continues to hold large parts of the surrounding region of the same name. Russian forces say they are making gains in the Donetsk region, although Kiev claims to have repelled their advances in some areas. The Ukrainian General Staff said soldiers had fought off Russian attacks north-east and east of Soledar and Bakhmut, while they said fighting continued south of Bakhmut. However, Brigadier General Olexiy Hromov admitted that Ukrainian troops there had had to withdraw from the village of Semyhirya. Hromov also said Ukrainian fighters had left their positions south-east of the industrial town of Avdiivka. Kiev's forces deflected Russian advances north of Sloviansk, west of Donetsk and in the Kherson and Kharkiv regions, the staff report said, despite Russian shelling of dozens of Ukrainian positions along the whole of the front line, alongside Russian aerial attacks. Russia is trying to seize power in the Donbas region, made up of Luhansk and Donetsk. While Moscow already controls Luhansk, it is now fighting to take the remainder of Donetsk. If places like Bakhmut were to fall to Russian forces, this would open the way for Russian troops to the outskirts of the twin cities Sloviansk and Kramatorsk. More than half a million people lived in the area before the war. It was not possible to verify the reports. Russian Defence Ministry spokesperson Igor Konashenkov said earlier that due to heavy losses, several Ukrainian brigades have withdrawn from their positions near Soledar, Avdiivka and Bakhmut. As Kiev continues to seek weapons to fight off Moscow's attacks, Ukraine received four Sukhoi Su-25 fighter jets from North Macedonia, according to reports. North Macedonia reportedly acquired the jets from Ukraine in 2001. The planes, for sale since 2003, had been deployed several times during this period, it said. North Macedonia also sent Ukraine Soviet-built T-72 tanks. Meanwhile, less than a week after the devastating attack on a prisoner-of-war camp in eastern Ukraine, UN Secretary General Antonio Guterres announced an investigation. The UN had received a request to that effect from both Russia and Ukraine, Guterres said in New York. He was not authorised to launch criminal investigations, but could initiate a fact-finding mission, the UN chief said. Preparations were underway. Dozens of Ukrainian prisoners of war were killed in the attack on the prison camp in Olenivka, near the city of Donetsk. The prison barracks were hit by a rocket on July 29, according to Russian sources. Moscow and Kiev blame each other for the attack. Kremlin spokesperson Dmitry Peskov said on Thursday that it was "absolutely proven" that Ukraine had killed its own soldiers who were being held as prisoners of war. "There is evidence of this and nothing can be hidden here." /IANS