Russian forces on Wednesday shelled eastern and southern Ukraine after the White House said it saw signs Moscow was preparing to formally annex territory it has seized during the nearly five-month war.

Ukraine's military and politicians reported heavy and sometimes fatal Russian shelling amid what they said were largely unsuccessful attempts by Russian ground forces to advance.

British military intelligence said Russia's offensive in the eastern Donbass region continued to make minimal gains as Ukrainian forces – which Britain helps support – held the line. More than two weeks have passed since Russia's last major territorial gain, the city of Lysychansk.

The British Defense Ministry said Wednesday that the bridge in Kherson was likely still usable after the Ukrainian strikes, but it is a "key vulnerability for Russian forces."

"It is one of only two road crossing points over the Dnieper by which Russia can supply or withdraw its forces in the territory it has occupied west of the river," it added. "Control of Dnieper crossings is likely to become a key factor in the outcome of fighting in the region."

Ukraine's presidential office said that at least 13 civilians were killed and a further 40 wounded by the Russian shelling across the country. At least five of the deaths were in the Donetsk region of Donbass.

"There is no safe place left in the region," Donetsk Governor Pavlo Kyrylenko said in televised remarks. "Residents should evacuate while they still can."

The Russian barrage also hit the Kharkiv region in northeastern Ukraine, where five people have been killed in the last 24 hours.

"These strikes targeting peaceful civilians make no sense, but the Russian army is continuing this senseless shelling," Kharkiv Governor Oleh Syniehubov said.

'Ukraine failed in deal'

Russian President Vladimir Putin said Wednesday that Moscow has not seen any moves from Ukraine to fulfill the terms of what he described as a preliminary peace deal agreed to in March.

Putin, speaking to reporters in televised comments after a visit to Iran, said Saudi Arabia and the United Arab Emirates (UAE) were offering to mediate between Russia and Ukraine.

There was no immediate response from the Ukrainian government to Putin's remarks in the early hours of Wednesday.

Putin, asked about a possible meeting with Ukrainian President Volodymyr Zelenskyy, said Kyiv had not stuck to the terms of a preliminary peace deal he said had been "practically achieved" in March, without elaborating.

"The final result of course ... depends on the willingness of the contracting parties to implement the agreements that were reached. Today we see the powers in Kyiv have no such desire," Putin said.

'Russia's objectives widen'

Meanwhile, the Kremlin has said there is no time limit to what it calls a "special military operation" to ensure its own security in the face of NATO's expansion and that it will do whatever it takes to achieve its aims.

Russian Foreign Minister Sergei Lavrov said the geographical objectives of Moscow's "special military operation" in Ukraine are no longer limited to the eastern Donbass region but include a number of other territories, Russian state news agency RIA Novosti reported Wednesday.

Lavrov added that Russia's objectives will expand still further if the West delivers long-range weapons to Kyiv.

Ukraine and the West condemn the conflict as an unprovoked war of aggression by Russia against its neighbor.

Five civilians were killed and 16 wounded by Russian shelling in the Donetsk region, while two civilians were killed by shelling in the city of Nikopol in the south, the respective Ukrainian regional governors said on Telegram.

Roman Starovoit, governor of Russia's Kursk region, which borders Ukraine, wrote on Telegram that Ukrainian forces had shelled a border crossing there.

Russia's Tass news agency, meanwhile, cited the mayor of Horlivka, a city in the Russian-backed self-proclaimed Donetsk People's Republic in eastern Ukraine, as saying that one person had been killed and three injured, including a child, by Ukrainian shelling.

Reuters could not immediately verify the Ukrainian or Russian accounts.

Russian Defense Minister Sergei Shoigu has inspected troops fighting in Ukraine, his ministry said Wednesday. It cited him as ordering more to be done to shoot down Ukrainian drones operating in border regions and to stop Ukraine shelling areas it has lost control of.

Citing U.S. intelligence, White House national security spokesperson John Kirby accused Russia of laying the groundwork to annex Ukrainian territory it had seized since the start of the war on Feb. 24, an assertion the Russian Embassy in Washington said mischaracterised what Moscow was trying to do.

The embassy cast what Ukraine and the West regard as a naked land grab as "returning peace to liberated territories" in order to protect the rights of people regardless of their ethnicity or language – a reference to Russian speakers or ethnic Russians in Ukraine who Russia says it is saving from persecution at the hands of what it has cast as dangerous Ukrainian nationalists.

On the diplomatic front, Russia's close ally Syria announced Wednesday it was severing ties with Ukraine, according to an unidentified Foreign Ministry official, cited by state news agency SANA.

Kyiv had already announced it was cutting ties with Syria late last month after Damascus recognized the Russian-backed breakaway republics of Donetsk and Luhansk in eastern Ukraine.