Air strikes hit Ukraine's strategic port Odessa

A soldier near the site of one of the strikes said it was likely a rocket or a missile

Explosions rocked the strategic Ukrainian port city of Odessa on Sunday, as a top UN official headed to Moscow to try to secure a "humanitarian ceasefire" and after evidence emerged of possible civilian killings around Kyiv.

Thick plumes of black smoke rose from several areas on the historic Black Sea port, after air strikes shook the city at about 6:00 am (0300 GMT) but the Ukrainian army said no one was killed.

Russia's defence ministry confirmed the attack, saying "high-precision sea and air-based missiles destroyed an oil refinery and three storage facilities for fuel and lubricants".

The ministry claimed the targets were supplying fuel to Ukrainian troops.

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Anton Herashchenko, an adviser to the Ukrainian interior minister, said: "Fires were reported in some areas. Some of the missiles were shot down by air defence."

The strikes came with Greece's Foreign Minister Nikos Dendias expected in the city to deliver humanitarian aid to the municipal authorities.

UN chief Antonio Guterres' humanitarian envoy Martin Griffiths was meanwhile seeking a halt in the fighting, which Ukraine estimates has left 20,000 people dead, and forced more than 10 million to flee for their lives.

He will fly on to Kyiv from Moscow. Both Russia and Ukraine have agreed to meet him, Guterres said on Saturday.

In the ravaged city of Bucha, just outside the Ukrainian capital, the bodies of nearly 300 civilians were found in mass graves after Russian troops withdrew.

AFP reporters saw at least 20 bodies, all in civilian clothing, strewn across a single street. One had his hands tied behind his back with a white cloth, and his Ukrainian passport left open beside his body.

"All these people were shot," Bucha's mayor Anatoly Fedoruk said, adding that 280 other bodies had been buried in mass graves in the town. "These are the consequences of Russian occupation."

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UK Foreign Secretary Liz Truss said she was "appalled by atrocities in Bucha and other towns in Ukraine" and promised the perpetrators would be held to account before an international war crimes tribunal.

The International Criminal Court has already opened a probe into possible war crimes committed in Ukraine, and several Western leaders, including US President Joe Biden, have accused Russia's Vladimir Putin of being a "war criminal".

Ukraine's President Volodymyr Zelensky has also alleged that Russian soldiers planted mines and other booby traps as they withdraw from northern Ukraine.

In a video address Saturday, he warned returning residents of tripwires and other dangers.

"We are moving forward. Moving carefully and everyone who returns to this area must also be very careful," he said.

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.

Pope Francis, on a visit to the Mediterranean island of Malta, issued a thinly veiled attack on Putin for ordering troops into Ukraine, and on Sunday made a plea for refugees fleeing the conflict to be welcomed.

The pontiff has not ruled out a visit to Kyiv.

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On talks to end the fighting, Ukrainian negotiator David Arakhamia told local television channels that Russia had "verbally" accepted most of Kyiv's proposals -- except on the issue of Crimea, which Moscow annexed in 2014.

Among the agreed-upon points was that a referendum on Ukraine's neutral status "will be the only way out of this situation", Arakhamia said.

He said any meeting between Zelensky and Putin would "with a high probability" take place in Turkey, which has sought to mediate the conflict.

As Russian forces withdraw from some northern areas, Moscow appears to be focusing on eastern and southern Ukraine, where it already holds vast swathes of territory.

UK Defence Intelligence said early Sunday that Russian air activity in the last week had been concentrating on southeastern Ukraine, "likely as a result of Russia focusing its military operations in this area".

But it said Russia was struggling to find and destroy air systems, which has "signficantly affected their ability to support the advance of their ground forces".

In his latest video message, Zelensky said Russian troops wanted to seize the disputed Donbas region and the south of Ukraine, promising "to defend our freedom, our land and our people".

But Ukrainian presidential adviser Mykhaylo Podolyak warned on social media that "without heavy weapons we won't be able to drive (Russia) out".

Ukraine authorities nevertheless offered citizens elements of good news Saturday in claiming progress against the Russians, more than five weeks after the start of Europe's worst conflict in decades.

"Irpin, Bucha, Gostomel and the whole Kyiv region were liberated from the invader," deputy defence minister Ganna Maliar said on Facebook, referring to towns that have been heavily damaged or destroyed by fighting.

Russia's efforts to consolidate its hold on southern and eastern areas of Ukraine have been hampered by the resistance of Mariupol despite devastating attacks lasting weeks.

At least 5,000 residents have been killed in the besieged southern port city, according to officials, while the estimated 160,000 who remain face shortages of food, water and electricity.

The International Committee of the Red Cross (ICRC) said its team left for Mariupol on Saturday to make another attempt at conducting an evacuation, after being forced to turn back the day before.

In another southern city, Enerhodar, which is under Russian control, a Ukrainian official said Russian forces opened fire on peaceful demonstrators, injuring four.

Zelensky has thanked the residents of Enerhodar, the site of Europe's largest nuclear power plant, which had been seized by Russian troops in early March, for their bravery.

"When people protest, and the more they protest, the harder it is for the occupiers to destroy us, to destroy our freedom," Zelensky said.

In Russia, hundreds of people gathered across the country Saturday to protest against the war in Ukraine. Police detained 211 people in several cities, according to OVD-Info, a group that monitors arrests.

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