An Estonian cargo ship has been sunk off the coast of Ukraine following an explosion, according to reports.

Igor Ilves, managing director of Tallinn-based manager Vista Shipping Agency told Reuters that two crew members were in a life raft at sea following the blast near Odessa.

Four crew members from the ship named The Helt are reportedly still missing.

Ives said: "The vessel has finally sunk.

"Two of the crew are in a raft on the water and four others are missing. I don't know where they are at the moment."

He added that the vessel may have been struck by a mine.

Estonia is a member of Nato and the sinking sparked fears it could lead to further conflict in Eastern Europe.

The US and its allies have insisted that Nato is a defensive alliance that doesn’t pose a threat to Russia.

Russian forces seize key Ukrainian port

It comes after latest figures from the UN show at least 227 civilians have been killed and another 525 wounded in the past week.

Russian forces have seized a strategic Ukrainian seaport and besieged another as part of efforts to cut the country off from its coastline.

The Russian military said it had control of Kherson, and local Ukrainian officials confirmed that forces have taken over local government headquarters in the Black Sea port of 280,000.

It makes Kherson the first major city to fall since the invasion began a week ago as Moscow said it was ready for talks to end the fighting that has sent more than one million people fleeing over Ukraine’s borders.

The Herald: Where Ukrainian refugees are seeking safety. (PA)Where Ukrainian refugees are seeking safety. (PA)

Elsewhere, the Russians pressed their offensive on multiple fronts, though a column of tanks and other vehicles has apparently been stalled for days outside the capital of Kyiv.

Heavy fighting continued on Thursday on the outskirts of another strategic port city on the Azov Sea, Mariupol, plunging it into darkness, isolation and fear. Electricity and phone connections were largely down, and homes and shops were facing food and water shortages.

On the far edges of Kyiv, volunteers well into their 60s manned a checkpoint to try to block the Russian advance.

“In my old age, I had to take up arms,” said Andrey Goncharuk, 68. He said the fighters needed more weapons, but “we’ll kill the enemy and take their weapons”.

Around Ukraine, others crowded into train stations, carrying children wrapped in blankets and dragging wheeled suitcases into new lives as refugees.

Among the million-plus refugees who have fled Ukraine in recent days were some 200 orphans with severe physical and mental disabilities who arrived from Kyiv by train in Hungary on Wednesday.

Some of them spent more than an hour in underground shelters during a bombing, said Larissa Leonidovna, the director of the Svyatoshinksy orphanage for boys.