Russia says it has detained eight people in connection with Saturday's explosion on a key bridge linking Russia to Crimea.

Its FSB security service said five of those held were Russians, while the others were Ukrainian and Armenian.

It says Kyiv was behind the attack but a Ukrainian official described Russia's investigation as "nonsense".

The news came as explosions were reported in the Ukrainian cities of Kherson, Zaporizhzhia and Nikopol.

The BBC's Hugo Bachega in Kyiv said five explosions had been heard in Kherson, one of the largest cities under Russian occupation, while there were unconfirmed reports that the air defence system in the city had been activated.

He said it was not clear what had triggered the explosions.

Ukraine's military said its troops were continuing their advance in the region, capturing another five settlements.

Meanwhile there were also blasts in several Ukrainian-controlled cities.

Three people, including a six-year-old, girl were seriously injured by shelling in Nikopol, in Dnipro region, a Ukrainian presidential spokesman said.

Ukraine's Emergency Ministry reported several S-300 missiles had fallen in and around Zaporizhzhia, with one destroying a residential building in a suburb. It said a family were pulled from the wreckage.

'Fake structures'

The blast on the Crimea Bridge was a powerful symbolic blow to Russian President Vladimir Putin, who opened the bridge in 2018, four years after Russia's annexation of Crimea.

President Putin called it an "act of terrorism", saying Ukraine's intelligence agents had aimed to destroy a critically important piece of Russia's civil infrastructure.

But a spokesman for the Ukrainian intelligence services, Andriy Yusov, rubbished the Russian accusations.

"All the activities of the FSB and [Russia's] Investigative Committee are nonsense," he told Ukrainian media. "They are fake structures which serve the

Putin regime, so we're definitely not going to comment on their latest announcements."

Russian forces retaliated on Monday with a wave of missile strikes across the country, including on central Kyiv, killing 19 people.

Following more strikes on Tuesday, Ukrainian President Volodymyr Zelensky urged countries to hit Moscow with more sanctions in response to "a new wave of terror".

He called on the West to find new ways to apply political pressure to Russia and support Ukraine.

The calls came after he met the G7 group of nations for emergency virtual talks on Tuesday.

The bloc - which consists of Canada, France, Germany, Italy, Japan, the UK and US - promised to continue providing "financial, humanitarian, military, diplomatic and legal" support to his country "for as long as it takes".

Nato also said it would stand with Ukraine for as long as necessary.

Separately International Atomic Energy Agency (IAEA) head Rafael Grossi tweeted that the Russian-occupied Zaporizhzhia nuclear plant had lost external power for the second time in five days and back-up diesel generators had been started up.

"This repeated loss of #ZNPP's off-site power is a deeply worrying development and it underlines the urgent need for a nuclear safety & security protection zone around the site," he added.

Also Ukrainian nuclear agency Enerhoatom said in a post on Telegram that the Russian authorities at the plant, the largest in Europe, were not allowing it to deliver fresh supplies of diesel fuel.

Moscow seized the massive facility in March, but kept on its Ukrainian staff. Both Ukraine and Russia accuse each other of repeatedly shelling the plant, amid global concerns that this could lead to a major radiation incident in Europe.

By Robert Greenall & Robert Plummer
BBC News