By Andrea Shalal and Pavel Polityuk

MYKOLAIV, Ukraine/KYIV (Reuters) - Ukrainian troops have attacked Russian positions along the entire front, Ukrainian President Volodymyr Zelenskiy said, while Russia halted gas supplies via a major pipeline to Germany citing the need for maintenance.

Separately, a team from the U.N. nuclear watchdog set off from Kyiv towards the Zaporizhzhia nuclear plant in Ukraine's south to assess any damage after shelling nearby sparked fears of a radiation disaster.

Ukraine, armed with sophisticated Western-supplied weapons, launched a fresh push this week to reclaim territory in its south. Zelenskiy, who urged Russian soldiers to flee for their lives, said his forces were also on the offensive in the east.

"Active military engagement is now happening along the whole front line: in the south, in the Kharkiv region, in Donbas," Zelenskiy said in his nightly address on Tuesday.

Russia captured large tracts of southern Ukraine near the Black Sea coast in the early weeks of the six-month-old war, including in the Kherson region, which lies north of the Russian-annexed Crimean Peninsula.   

    Ukraine sees recapturing the region as crucial to prevent Russian attempts to seize more territory further west that could eventually cut off its access to the Black Sea.

Kyiv has released few details of the offensive.

Britain, an ally of Ukraine, said Ukrainian formations in the south had pushed Russian front-line forces back some distance in places, exploiting relatively thin Russian defences.

While it was not possible to confirm the extent of Ukraine's advances, unverified reports, images and footage on social media suggest its forces may have taken back some villages and destroyed some Russian targets in the south.

    Russia's defence ministry, however, said its troops had routed Ukrainian forces, adding that air defence units had shot down dozens of missiles near Kherson.

Russia is methodically pressing on with its plans in Ukraine, Kremlin spokesman Dmitry Peskov said on Tuesday.

"All of our goals will be reached," he said.

Reuters could not verify the battlefield reports.

'EXTREMELY DANGEROUS'

The Zaporizhzhia nuclear power plant, captured by Russian troops in March but still manned by Ukrainian staff, has been a hotspot in the conflict with both sides trading blame for shelling in the vicinity.

"The situation at the Zaporizhzhia nuclear power plant and in Enerhodar and surrounding areas remains extremely dangerous," Zelenskiy said. "The risk of a radiation disaster due to Russian actions does not decrease for an hour."

The Russian defence ministry, however, said radiation levels were normal at the plant, Europe's largest.

The Interfax news agency quoted a Russian-appointed Zaporizhzhia government official as saying on Wednesday that two of the plant's six reactors were running.

The mission from the U.N. nuclear watchdog, International Atomic Energy Agency (IAEA), led by its chief, Rafael Grossi, is expected to visit the plant this week for inspections.

"We are now finally moving after six months of strenuous efforts," Grossi said before the IAEA convoy set off from Kyiv, adding they planned to spend a few days at the site.

"We have a very important task there to perform - to assess the real situations there, to help stabilise the situation as much as we can," he told reporters.

Yevgeny Balitsky, the head of the Russian-installed administration, earlier told Interfax that the IAEA inspectors "must see the work of the station in one day".

Ukraine on Tuesday accused Russia of shelling a corridor that IAEA officials would need to use to reach the plant in an effort to get them to travel via Russian-annexed Crimea instead.

There was no immediate response from Moscow.

Ukraine's military said on Wednesday Russian forces were using tanks, rockets and artillery along a contact line in the area.

FLOW TO ZERO

Russia invaded Ukraine on Feb. 24 for what it calls a "special military operation" to demilitarise its neighbour.

Ukraine and the West describe it as an unprovoked war of aggression that has killed thousands, displaced millions, created food shortages and pushed up energy prices amid unprecedented Western sanctions.

Natural gas prices have soared in Europe to all-time highs as Russia has been pumping gas via Nord Stream 1 pipeline at only 20% of capacity, citing equipment problems.

On Wednesday, flows fell to zero in the pipeline to Germany via the Baltic Sea, as Russia's Gazprom (MCX:GAZP) shut it for maintenance.

The state energy giant has said the latest 72-hour shutdown was needed for maintenance on the pipeline's only remaining compressor. Germany has said there were no technical issues.

European governments fear Russia could extend the outage in retaliation for sanctions imposed on it and have accused President Vladimir Putin of using energy supplies as a weapon.

Russia has denied that and blames sanctions for the drop in exports.

France's energy minister said Gazprom was using an excuse to switch off gas deliveries to its French contractor but added that France had anticipated the loss of supply.

Gazprom said earlier it was suspending gas deliveries to French utility Engie citing a dispute over payments.