Nato “neutrality patrols” could help keep supply routes open for Ukraine, a defence expert has said, after an ambassador invited the alliance to send ships to “unblock” the Black Sea from Russian vessels.

The Ukrainian ambassador to the UK Vadym Prystaiko said Nato countries needed to take a more active role in opening up the Black Sea south of Ukraine to cargo ships, saying it would help humanitarian aid reach the country.

He also stressed that grain exports from the southern port city of Odesa – the busiest port in Ukraine – is important for food security around the world.

Gustav Gressel, a former Austrian defence official and Russian military expert at the European Council on Foreign Relations, told i that “neutrality patrols”, such as those seen in the Second World War, would help keep trade routes open.

In the earlier years of the Second World War, the neutrality patrol saw US warships assisting the UK in convoying merchant vessels across the Atlantic Ocean.

Despite claiming to be neutral, the US was clearly favouring the British by conducting battleship sweeps deep into the Atlantic to deter German merchant raiders and U-boats from entering neutral waters. The Americans radioed their location without encryption so that they could be intercepted by the Royal Navy.

“I think neutrality patrols in the Black Sea would be a good thing,” Mr Gressel said. “Also to protect Romania’s and Bulgaria’s trade routes.”

However, the Montreux Convention, which gives Turkey certain powers over the Bosporus and Dardanelles Straits linking the Mediterranean Sea and the Black Sea, limits the international naval presence in the Black Sea. A non-Black Sea country’s ships cannot exceed a total of 30,000 tonnes at any time, and can only stay in the region for 21 days.

“They (Nato) are welcome in our sea,” Mr Prystaiko told Sky News on Friday. “They are invited to Odesa.”

He added: “We have to unblock the Black Sea, Russians are blocking the sea for us, that is our own sea, we need it for humanitarian assistance to come into Ukraine and we have it to allow Ukrainian exports, mostly grain.

“Many nations in Africa and Asia are fed because of Ukrainian grain, and this grain is stuck in our silos and can’t be exported, can’t be sent anywhere.”

He said there were already three Nato members in the Black Sea – Turkey, Bulgaria and Romania – and so it is “not a big deal”.

Fighting in southern and eastern Ukraine has intensified after Russian forces retreated from the north.

Ukraine’s military said Russia wants to entrench a land passage between the eastern separatist Donbas region and the southern region of Crimea that Moscow seized and annexed in 2014.

Oleh Synyehubov, the governor of Kharkiv in eastern Ukraine, said heavy Russian shelling was intended to “put pressure on the civilian population” and destroy civilian infrastructure.