The first grain ship from Ukraine since Russia's invasion is ready and waiting for the signal to leave port, says President Volodymyr Zelensky.

He was speaking on a visit to the port of Chornomorsk, a week after a UN-led agreement to resume exports.

The blockade of Ukrainian ports has led to food shortages and price rises.

While significant, the first departure from Ukraine's shores will be more of a testing of the waters than an unblocking of a major supply route.

With hopes of an imminent departure, President Zelensky joined ambassadors from the G7 industrialised nations on the Black Sea coast, standing in front of the Turkish-registered ship Polarnet.

Chornomorsk, south of Odesa, has seen the first shipping activity in weeks, with tugboats manoeuvring and a single vessel changing position.

It is one of three Black Sea ports waiting for the green light. Officials say 17 ships are now waiting with 600,000 tonnes of cargo.

UN aid chief Martin Griffiths made clear exports could only resume safely when the route through the Black Sea was finalised. By the admission of global shipping insurers and the UN, "crucial" details still had to be ironed out and nothing was expected to happen before the weekend.

An added risk to the operation is that the biggest port of Odesa has been mined by Ukraine's military.

If and when the first ship does finally leave, convoys of grain will not immediately follow.

Under the agreement signed by Russia and Ukraine, the sea corridor, convoy and inspection of the cargo are all being organised by a joint co-ordination centre (JCC) in Turkey and final preparations were reportedly still not in place.

The suspension of grain exports, now into a sixth month, has led to food shortages across Africa.

The destination of the first ship was not yet clear, but the UN aid chief said Somalia was a priority. Eight areas of the country are at risk of famine.

Seven ambassadors from the G7 joined the Ukrainian president to reflect the political will to get exports going again. "While someone, blocking the Black Sea, takes the lives of other countries, we are giving them opportunity to survive," Mr Zelensky said.

Russian naval forces control most of the Black Sea, leaving some 20 million tonnes of grain stored in Ukraine, waiting for export.

Before Russia's invasion, the two countries accounted for a third of the world's exports of wheat and barley. There is optimism that the deal, set for an initial 120 days, may work. Russia is also keen to resume its own exports of grain and fertiliser.

The regional head in Odesa, Serhiy Bratchuk, posted a map showing how the grain corridor to and from Odesa might operate, with an area for inspection near the port and a route following the Ukrainian coast to the mouth of the River Danube.

Ukrainian officials said Chornomorsk and Odesa were ready for ships to leave, while a third port, Pivdennyi, would be prepared by the end of the week.

Foreign Minister Dmytro Kuleba said all the details of the first shipment had to be calculated and everything "checked once more, twice, three times".

The UN aid chief said it could only take place safely when the parties working at the co-ordination centre in Istanbul had agreed to the precise location of the corridor as well as the movements.

But he believed any problems would be settled very quickly, with the aim of returning to pre-war export levels of some five million metric tonnes a month.