WRAPUP 9-Russia pummels eastern towns in bid to encircle Ukraine forces

(Adds late-night comments from Ukrainian president)

* Russian forces advance in east, shifting momentum

* Capture of Lyman would set stage for next phase of offensive

* Sievierodonetsk under assault

By Max Hunder and Conor Humphries

KYIV, May 28 (Reuters) - Russian forces stepped up their assault on the Ukrainian city of Sievierodonetsk on Saturday after claiming to have captured the nearby rail hub of Lyman, as Kyiv intensified its calls for longer-range weaponry from the West to help it fight back in the Donbas region.

Slow, solid Russian gains in recent days point to a subtle momentum shift in the war, now in its fourth month. The invading forces appear close to seizing all of the Luhansk region of Donbas, one of the more modest war goals the Kremlin set after abandoning its assault on Kyiv in the face of Ukrainian resistance.

Russia's defence ministry said on Saturday its troops and allied separatist forces were now in full control of Lyman, the site of a railway junction west of the Siverskyi Donets River in the Donetsk region that neighbours Luhansk.

However, Ukraine's deputy defence minister, Hanna Malyar, said the battle for Lyman continued, the ZN.ua website reported.

Sievierodonetsk, some 60 km (40 miles) from Lyman on the eastern side of the river and the largest Donbas city still held by Ukraine, was under heavy assault from the Russians.

"Sievierodonetsk is under constant enemy fire," Ukrainian police posted on social media on Saturday.

Russian artillery was also shelling the Lysychansk-Bakhmut road, which Russia must take to close a pincer movement and encircle Ukrainian forces.

"There was significant destruction in Lysychansk," the police said.

The governor of Luhansk, which along with Donetsk comprises the Donbas, said on Friday Russian troops had already entered Sievierodonetsk. Ukrainian troops may have to retreat from the city to avoid capture, Governor Serhiy Gaidai said. It was not clear whether they had begun to pull out on Saturday.

Ukrainian presidential adviser and peace talks negotiator Mykhailo Podolyak on Saturday repeated a call for deliveries of U.S.-made long-range multiple-rocket launchers. U.S. officials tell Reuters such systems are actively being considered, with a decision possible in the coming days.

"It is hard to fight when you are attacked from 70 km away and have nothing to fight back with. Ukraine can return Russia behind the Iron Curtain, but we need effective weapons for that," Podolyak tweeted.

President Volodymyr Zelenskiy voiced hopes in a late-night video address that allies would provide needed weaponry and added that he expected "good news on this next week."