Ukraine on Wednesday said it had restarted operations at its blockaded Black Sea ports as it moved closer to resuming grain exports with the opening of a coordination centre to oversee a UN-backed deal.

Progress towards fulfilling the landmark agreement came as Kyiv's artillery struck a key bridge in Moscow-controlled territory in south Ukraine, damaging an important supply route as Ukrainian forces look to wrest back the Kherson region.

And as German authorities said Russia drastically reduced gas deliveries to Europe in a move seen as revenge for Western sanctions over the invasion, Ukraine announced plans to increase its electricity imports to Europe.

Ukraine and Russia last week agreed a plan with the help of Turkey and the United Nations to allow grain stranded by Moscow's naval blockade to be exported from three ports.

Kyiv has said it hopes to begin sending out the first of millions of tonnes of grain this week despite a missile strike by Russia over the weekend on the port in Odessa.

Ukraine's navy said "work has resumed" at the export hubs to prepare for ships to be escorted through the mine-infested waters to reach world markets.

As part of the deal, a coordination centre involving Ukrainian and Russian representatives opened in Istanbul to monitor the safe passage for shipping along established routes and oversee inspections for banned weapons.

The blockage of deliveries from two of the world's biggest grain exporters has contributed to a spike in prices that has made food imports prohibitively expensive for some of the world's poorest countries.

- 'Leave Kherson' -

Fighting has continued to rage on the ground in Ukraine despite the push to get the grain out, and Kyiv struck back by hitting the vital Antonivskiy bridge over the Dnipro river in a move that threatens to cut supply lines to Russian troops.

Ukraine's Defence Ministry said on Twitter the strikes on bridges over the Dnipro created an "impossible dilemma" for Russia: "retreat or be annihilated by the Ukrainian army".

Kirill Stremousov, the deputy head of the Russian-installed regional administration in Kherson, confirmed the bridge had been hit overnight and traffic had been halted.

But he sought to downplay the damage, insisting that the attack would not affect the outcome of the hostilities "in any way".

Ukrainian forces in recent weeks have been clawing back territory in the Kherson region, which fell to Russian forces easily and early after their invasion launched on February 24.

Their counter-offensive, supported by Western-supplied long-range artillery, has seen its forces push closer to Kherson city, which had a pre-war population of under 300,000 people.

Russian forces "should leave Kherson while it is still possible. There may not be a third warning," Ukrainian presidential advisor Mykhaylo Podolyak said on Twitter after the attack.

Meanwhile the leader of pro-Russian separatists in eastern Ukraine called on Moscow to conquer key cities across the country.

"Today the time has come to liberate Russian cities, founded by Russians: Kyiv, Chernigiv, Poltava, Odessa, Dnipro, Kharkiv, Zaporizhzhia, Lutsk," Denis Pushilin said on Telegram.

Russia sought to capture the capital Kyiv in the early days of its invasion but later retreated, focusing its efforts on Ukraine's east Donbas region.

In the battered Donetsk region -- part of the Donbas -- AFP journalists saw a house hit in an intense artillery exchange around the ravaged frontline city of Bakhmut.

A worker was inside the courtyard when the shell hit and was saved by emergency rescuers who cut a hole in a steel fence using an axe.

"I heard a whistle. And I don't remember anything. It exploded and I was thrown into the barn by the explosion," 51-year-old Roman told AFP.

Ukraine's emergency services said that Russian artillery had hit a hotel in Bakhmut, leaving two people dead and five injured.

- Gas 'power play' -

Deepening an energy crisis in Europe sparked by the war, Germany's energy regulator said flow of Russian gas via the key Nord Stream pipeline had dropped to 20 percent of capacity on Wednesday from 40 percent.

Kremlin spokesman Dmitry Peskov blamed EU sanctions for the limited supply, but Berlin has dismissed the explanation and government spokeswoman Christiane Hoffmann called the reductions a "power play" by Moscow.

The European Union has been bracing for energy cutbacks and on Tuesday agreed a plan to reduce gas consumption by 15 percent this winter to break its dependence on Russia.

Reacting to Europe's energy concerns, Ukrainian President Volodymyr Zelensky announced plans to boost Ukrainian electricity supplies to European consumers.

"Our export not only allows us to generate foreign currency revenues, but also help our partners withstand the energy pressure from Russia," Zelensky said in his daily address to the nation Wednesday evening.