KYIV, Ukraine (AP) — Fires burned and ammunition exploded at a depot in Crimea on Wednesday, a day after the latest suspected Ukrainian attack on a military site in the Russia-annexed peninsula, highlighting the challenges facing Moscow.

The peninsula, which Russia seized in 2014, was once a secure base that Moscow's forces have used to launch attacks — and it was a staging ground for the start of the Feb. 24 invasion. But in recent days, explosions have destroyed several Russian planes at an air base in Crimea, and munitions blew up Tuesday.

Ukrainian authorities have stopped short of publicly claiming responsibility, but President Volodymyr Zelenskyy alluded to Ukrainian attacks behind enemy lines after the most recent blasts Tuesday while Russia blamed "sabotage."

The spate of attacks represented the latest setback for Moscow, which began its invasion with hopes of taking the capital of Kyiv and much of the country in a lightning blitz but soon became bogged down in the face of fiercer than expected resistance from Ukrainian forces.

As the war nears the half-year mark, the sides are now engaged in a war of attrition, fighting village to village, largely in the country's east. The attacks in Crimea could open a new front that would represent a significant escalation in the war and further stretch Russia's military resources.

"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," Britain's Defense ministry wrote on Twitter.

But it was not clear whether the attacks in Crimea would unblock the stalemate, as Ukrainian and Russian forces grind each other down in a war that has driven millions from their homes, disrupted food supplies worldwide and occasionally raised concerns about a nuclear accident.

On Thursday, U.N. Secretary-General Antonio Guterres plans to travel to Ukraine for a meeting with Zelenskyy and Turkish President Recep Tayyip Erdogan to discuss getting out grain shipments that are critical to feeding the world's hungry. They are also expected to talk about a possible fact-finding mission to the Russian-controlled Zaporizhzhia nuclear power plant, which Moscow and Kyiv have accused each other of shelling.

Tuesday's explosions and fires ripped through an ammunition depot near Dzhankoi in Crimea, leading to chaotic scenes when around 3,000 people had to be evacuated.

As a vivid reminder of Russia's vulnerability in Crimea, the peninsula's regional leader, Sergei Aksyonov, said that authorities were still fighting the fires Wednesday with a helicopter, as minutions continued to detonate. He said that a search for perpetrators of the attack was underway.

The Kommersant business paper also reported explosions Tuesday at a base in Gvardeyskoye. By Wednesday, there still was no comment from the Russian authorities.

The British intelligence report noted that Gvardeyskoye and Dzhankoi "are home to two of the most important Russian military airfields in Crimea."

A week earlier, Russia's military came under pressure on the peninsula when Ukraine said nine Russian warplanes were destroyed following explosions at Crimea's Saki air base. The massive explosions sent plumes of smoke rising over nearby beaches and caused sunbathers to flee.

At the time, Moscow suggested that the blasts were accidental, perhaps caused by a careless smoker, an explanation that drew mockery from Ukrainian authorities who hinted at their involvement in the attack but didn't directly claim responsibility.

On the eastern front, the stalemate continued, with the shelling causing ever more death and destruction.

In the Donetsk region that is the current focus of the Russian offensive, two civilians were killed and seven others were wounded by recent Russian shelling of several towns and villages.

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

In Mykolaiv, also in the south, two Russian missiles damaged a university building early Wednesday but injured no one.

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

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