According to US and Russian authorities, Ukrainian forces increased their artillery bombardment and ground assaults in a frenzy of aggressive military action that was raging throughout various sectors of the front line by Monday, The New York Times reported.

After months of planning a "counteroffensive", Ukraine has kept quiet regarding the details of its military moves. American officials who spoke on the condition of anonymity say the increase in strikes might be a hint that Kiev's counteroffensive has already begun, as per NYT. 

The authorities' conclusion was based in part on data from US military satellites, which identified an increase in activity from Ukrainian military sites. The satellites can follow artillery fire and missile launches using infrared technology.

This comes as the Ukrainian armed forces launched a large-scale offensive in five sectors of the South Donetsk area on Sunday, according to the Russian Defense Ministry Spokesperson Lt. Gen. Igor Konashenkov.

He said, "On the morning of June 4, the enemy launched a large-scale offensive in five sectors of the front in the South Donetsk direction by introducing into battle the 23rd and 31st mechanized brigades from the strategic reserves of the armed forces of Ukraine with the support of other military units and divisions."

On its part, Ukraine claimed on Monday that its troops were advancing around Bakhmut while playing down the importance of its "offensive actions", with Russia saying it had repelled a large-scale attack.

"The Bakhmut sector remains the epicentre of the hostilities. We are advancing there on a rather wide front," Ukrainian Deputy Defense Minister Ganna Malyar said.

"We are having some success," he added.

Earlier on Sunday, the Russian Defense Ministry said an artillery strike dispersed a Ukrainian group that attempted to carry out an operation in western Russia near the town of Novaya Tavolzhanka.

Ukrainian authorities have yet to respond.

The New York Times cited American military analysts as saying that Ukrainian battalions have made an initial push to ascertain the position and strength of Russia's forces.

Failure, or a lack of rapid progress, might impede Ukraine's capacity to obtain greater Western support and complicate Kiev's drive for extra security assurances at the NATO summit this summer, as per the newspaper.

The early reports of combat Sunday night and Monday morning were of bigger unit movements, which might indicate that Ukraine has pushed its NATO-trained soldiers into the conflict.

Ukrainian leaders have not notified their American colleagues when the counteroffensive would begin but have given them a time window within which they aim to launch their offensive against Russia.

On Sunday, the Ukrainian Ministry of Defense issued an elaborate film in which it stated that it would keep silent about the commencement of an expected counteroffensive against Russia, which it had been planning for months. It was unclear if Colonel-General Oleksandr Syrsky's tweet about Bakhmut was part of a larger assault or more isolated and restricted combat in a struggle for the city that has been raging since last summer.

Last month, the Russian Armed Forces said they gained full control over the city of Bakhmut, also known as Artyomvsk.