KYIV (Reuters) -Ukrainian forces have stopped Russian troops advancing in the northeastern Sumy region and are now battling to regain control along the border with Russia, President Volodymyr Zelenskiy said.

In remarks released for publication by his office on Saturday, Zelenskiy said that Moscow has amassed about 53,000 troops in the direction of Sumy.

"We are leveling the position. The fighting there is along the border. You should understand that the enemy has been stopped there. And the maximum depth at which the fighting takes place is 7 km (4.35 miles) from the border," Zelenskiy said.

Reuters could not verify the battlefield reports.

Russia's troops have been focusing their assaults in the eastern Donetsk region. But since the start of the month, they have intensified their attacks in the north-east, announcing plans to create a so-called 'buffer zone' in the Sumy and Kharkiv regions.

The Russian war in Ukraine is in its fourth year, but it has intensified in recent weeks.

Ukraine conducted an audacious drone attack this month that took out multiple aircraft inside Russia and also hit the bridge connecting Russia to the annexed Crimean peninsula using underwater explosives.

Moscow ramped up its air assaults after the attack.

Zelenskiy said that Ukrainian troops had maintained their defensive lines along more than 1,000 kilometres of the frontline. He also dismissed Moscow's claims that Russian troops had crossed the administrative border into the central Ukrainian region of Dnipropetrovsk.

Zelenskiy said that Russia was sending small assault groups "to get one foot on the administrative border" and make a picture or a video, but these attacks were repelled.

Dnipropetrovsk borders three regions that are partially occupied by Russia – Donetsk, Kherson and Zaporizhzhia. Russia now controls about one-fifth of Ukrainian territory.

Zelenskiy acknowledged that Ukraine was unable to regain all of its territory by military force and reiterated his pleas for stronger sanctions on Russia to force Moscow into negotiations to end the war.

Two rounds of peace talks between Kyiv and Moscow in Istanbul produced few results that could lead to a ceasefire and a broader peace deal. The two sides agreed only to exchange prisoners of war.

Several swaps have already been conducted this month, and Zelenskiy said he expected them to continue until June 20 or 21.

In separate remarks made on communications platform Telegram on Saturday, he said that a new group of Ukrainian prisoners of war had come home as part of another swap with Russia.

"We continue to take our people out of Russian captivity. This is the fourth exchange in a week," Zelenskiy wrote on his personal account.

Ukrainian officials responsible for exchanging prisoners said the vast majority of the soldiers released in the exchange had been held captive since 2022 with many captured during the defence of Mariupol.

The officials said Kyiv had, meanwhile, received the bodies of 1,200 of its soldiers killed in the war with Russia. The bodies were handed over to Ukraine on Friday.

"The agreement is that the exchanges will be completed, and the sides will discuss the next step," Zelenskiy said.

(Reporting by Olena Harmash; Editing by Toby Chopra and Joe Bavier)