KYIV, June 3 - Russia is pummeling major Ukrainian cities in an effort to sustain a sputtering war effort marked by a struggle to maintain momentum on the battlefield, where advances over the last month slowed to a near halt, researchers say.

Tuesday's devastating air attacks on Ukraine - which killed 23 people and wounded more than 130 - followed months of stalling progress for Moscow, which has faced stiffer Ukrainian resistance on the ground and mounting strikes on oil infrastructure and military-industrial sites from the air.

While Russian forces are still grinding forward in parts of Ukraine's eastern region of Donetsk, their glacial pace - combined with pushback elsewhere on the front line - threatens to weaken the Kremlin's hand in any future peace talks, analysts say.

President Vladimir Putin has insisted that the war cannot end unless Russia controls all of Donbas - which groups the Ukrainian regions of Donetsk and Luhansk. Ukrainian forces still hold around a fifth of Donetsk.

"In the broader picture, if the Russians can't find ways to pick up momentum significantly, the goal of capturing Donbas this year is slipping out of their reach fast," said John Helin, analyst and co-founder of the Black Bird Group, a Finnish conflict-analysis team.

According to Black Bird Group's latest data, shared with Reuters, Russian forces captured just 82 square km of Ukrainian territory in May, compared to 94 square km in April and 25 square km in March.

That stands in stark contrast to the much larger gains made over the same period last year, when Moscow's troops took 538 square km, 226 square km, 185 square km, respectively, according to Black Bird.

The Institute for the Study of War (ISW), a Washington-based think tank, said a series of escalating Russian strikes on Kyiv and other cities in recent weeks has been aimed partly at distracting from Moscow's battlefield woes and the impact of Ukraine's own long-range attacks inside Russia.

On Wednesday, Ukrainian drones struck an oil export terminal in St Petersburg, hours before a major annual economic forum...