A spate of fires and explosions has turned Russian-annexed Crimea from a secure base for the further invasion of Ukraine into the latest flashpoint highlighting Moscow's challenges ahead in a war that is nearing the half-year mark.

A statement from British defense intelligence Wednesday that “Russian commanders will highly likely be increasingly concerned with the apparent deterioration in security across Crimea, which functions as rear base area for the occupation”.

Even Russia itself acknowledged it was an “act of sabotage” that caused Tuesday's explosions and fires that ripped through an ammunition depot near Dzhankoi in once-secure Crimea, leading to chaotic scenes when around 3,000 people had to be evacuated.

As a vivid reminder of Russia's vulnerability in Crimea, detonations at the depot near Dzhankoi were still continuing Wednesday.

A week earlier, Russia's military in Crimea already came under pressure when Ukraine said nine Russian warplanes were destroyed following explosions. At the time, Moscow still offered the possibility of a wayward cigarette butt as the cause.

No such explanations would suffice anymore as the war, which had long centered on brutal fighting in Ukraine's eastern Donbas region, has now given southern Crimea increasing importance.

Worsening the outlook in Crimea was a report by the Kommersant business paper, that explosions had also taken place near Gvardeyskoye in the centre of the peninsula. By Wednesday, there still was no comment from the Russian authorities.

The British intelligence report said Gvardeyskoye and Dzhankoi “are home to two of the most important Russian military airfields in Crimea”.

Ukraine has stopped short of claiming responsibility for any of the blasts, including those at another Crimean air base last week. Russia seized the Crimean Peninsula in 2014 and has used it to launch attacks against Ukraine in the war that began on Feb 24.

If Ukrainian forces were behind the explosions, that would represent a significant escalation in the war. Such attacks could also indicate that Ukrainian operatives are able to penetrate deeply into Russian-occupied territory.

On the eastern front, the stalemate between both sides continued, with the brutality of the shelling causing ever more death and destruction.

In the Donetsk region at the forefront of the Russian offensive, two civilians were killed and seven others were wounded by recent Russian shelling of several towns and villages.

Russian Tu-22M3 long-range bombers fired cruise missiles at the Odesa region overnight, leaving four people injured, according to Odesa regional administration spokesman Oleh Bratchuk.

In the southern city of Mykolaiv, two Russian missiles damaged a university building early Wednesday but injured no one.

The Russian forces also shelled Kharkiv and various parts of the Kharkiv region overnight, damaging residential buildings and civilian infrastructure but inflicting no casualties.

On Thursday, UN Secretary-General Antonio Guterres plans to travel to Ukraine for a meeting in the western city of Lviv with Zelenskyy and Turkish President Recep Tayyip Erdogan. They are expected to discuss the grain shipments and a possible fact-finding mission to the Russian-controlled Zaporizhzhia nuclear power plant, which Russia and Ukraine have accused each other of shelling.