An explosion is seen in this undated handout video. Reuters

A series of explosions set off fires Sunday morning in Ukraine's southwestern city Odessa, according to an AFP journalist.

At around 6am (0300 GMT) the blasts sent up at least three columns of black smoke with flames visible apparently in an industrial part of the strategic Black Sea port.

Anton Herashchenko, adviser to the interior minister, said Odessa was attacked from the air.

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"Fires were reported in some areas. Part of the missiles were shot down by air defence," he wrote on Telegram.

Meanwhile, in an attempt to raise economic pressure on Russia, the Baltic states of Latvia, Estonia and Lithuania announced Saturday that they had stopped all imports of Russian natural gas.

Destroyed homes attacked by Russian forces are pictured in the village of Bachtanka, Odessa. AFP

Meanhile, residents of Ukraine’s besieged southeastern coast awaited possible evacuation Sunday as the country’s president said Russia’s obsession with capturing a key port city had left it weakened and created opportunities for his military.

With Mariupol squarely in Russia’s crosshairs, Ukraine insists it has gained a leg up elsewhere in the country, leading to troops retaking territory north of the capital of Kyiv as Russian forces departed.

"Ukraine has gained invaluable time, time that is allowing us to foil the enemy’s tactics and weaken its capabilities,” President Volodymyr Zelenskyy said late Saturday.

The interior of a house destroyed by Russian forces in the village of Bachtanka, Odessa, Ukraine. AFP

Inside Mariupol, though, surrounded by Russian forces for more than a month and brutalised by some of the war’s worst attacks, conditions remain dire and prospects for escape uncertain.

About 100,000 people are believed to remain in the Sea of Azov city, less than a quarter its prewar population of 430,000, and dire shortages of water, food, fuel and medicine persist.

Many still in Mariupol await fulfillment of promises to help them reach safety. Among those trying to get residents out was Russian forces had blocked the International Committee of the Red Cross, which still hadn’t reached the city on Saturday, a day after local authorities said it.