Ukrainian forces have stepped up their counteroffensive to regain lost territory in the south, as Russia launched deadly strikes in central Ukraine and near the capital amid reports that Moscow has been increasingly facing a shortage of personnel in the east.

Ukraine's military said its planes struck five Russian strongholds around the city of Kherson and another nearby city in the south, where it is concentrating its biggest counteroffensive since the start of the war, seeking to isolate the Russian troops in the area.

Ukraine has used Western-supplied long-range missile systems to badly damage three bridges across the Dnieper River in recent weeks, making it more difficult for Russia to supply its forces on the western bank.

The Kherson region, which borders Moscow-annexed Crimea, fell to the Russians soon after the the February 24 invasion.

Russian strikes on the central Ukrainian region of Kirovograd on July 28 killed five people and wounded 26 in the city of Kropyvnytskiy, officials said.

Near Kyiv, 15 people were hurt at a military base. Ukraine's northern and southern regions were also hit.

Russian missile strikes also hit the regions of Kyiv, Chernihiv, Mykolayiv, and Kharkiv. The attack on the Kyiv region -- the first in weeks -- hit a military unit in a village on the outskirts of the capital, according to Oleksiy Hromov, a senior official with Ukraine’s General Staff.

Fifteen people were wounded in the strikes, five of them civilians, Kyiv regional Governor Oleksiy Kuleba said.

Chernihiv regional Governor Vyacheslav Chaus reported that the Russians fired missiles from Belarus at the village of Honcharivska. The Chernihiv region had not been targeted in weeks.

There also has been heavy shelling along the entire front line in the eastern Donetsk region as Russian forces targeted Bakhmut and other cities. Three people died in Bakhmut, local officials said.

Britain's Defense Ministry said in its daily intelligence bulletin on July 29 that Moscow has been using mercenaries from the notorious Kremlin-linked Vagner Group in eastern Ukraine from the early days of the invasion alongside regular army units "in coordination with the Russian military."

Vagner, which is believed to be controlled by Yevgeny Prigozhin, a close associate of Russian President Vladimir Putin, has been involved in covert operations in Africa and the Middle East since 2015.

The use of mercenaries in overt military operations further undermines the Kremlin’s longstanding policy of denying links between private military contractors and the Russian state, British intelligence said in its report.

While the use of Vagner mercenaries in regular military operations was probably prompted by a "major shortage of combat infantry," British intel suggested that it is "highly unlikely" they would significantly influence the course of the war.

A U.S. lawmaker said on July 28 that more than 75,000 Russian soldiers -- about half the force sent by Moscow to invade Ukraine in February -- are believed to have been killed or wounded.

Representative Elissa Slotkin (Democrat-Michigan) who spoke to CNN after attending a classified briefing with officials from President Joe Biden's administration, said the figure was "huge."

Military casualties are a state secret in Russia even in peace time, and there are no updated official figures available on Moscow's military's death toll. The most recent CIA estimate was that 15,000 Russian forces had been killed in fighting and three times that number wounded.

Slotkin, who recently returned from a trip to Ukraine, said the next three to six weeks could be crucial for the direction the conflict would take.

"I think that what we heard very firmly from President Zelenskiy and reinforced today is that the Ukrainians really want to hit Russia in the teeth a few times before the winter comes, put them in the best position possible, particularly hitting them down south," Slotkin said.

U.S. Ambassador to NATO Julianne Smith told RFE/RL in an interview that the Ukrainian military commanders can determine themselves the next steps in the counteroffensive, but NATO wants to hear which systems would be most useful.

“We will continue to do everything we can to support Ukrainian military on the ground and make sure that they have what they need so that they can prevail in this conflict and stop Russian aggression,” she said.

With reporting by Reuters, BBC, and AP