Volodymyr Zelenskyy has said Ukraine could not agree to a Russian proposal to give up more of his country’s territory in exchange for a ceasefire because Moscow would use what it gained as a springboard to start a future war.

The Ukrainian president said he did not believe that Donald Trump supported Russia’s demands, and he expressed hope the US leader would act as an honest mediator when he meets Vladimir Putin in Alaska on Friday.

He added there was no sign that Russia was preparing to implement a ceasefire, as reports emerged that small sabotage groups had pierced Ukrainian defences in the eastern Donbas, advancing about six miles in three days. Zelenskyy also warned that Russia was planning new offensives on three parts of the frontline.

Speaking to journalists in the run-up to the Trump-Putin summit, and a day before a virtual meeting with US and European leaders, Zelenskyy said he believed Putin wanted to dominate his country because he “does not want a sovereign Ukraine”.

It was therefore dangerous, Zelenskyy said, for Ukraine to be forced by the US into accepting Russia’s demand to take over the parts of Donbas it does not control after the Alaska summit. The region sought by Russia amounted to “about 90,000 square kilometres” of the country, he said.

Last week Russia indicated it was prepared to consider a ceasefire in the Ukraine war for the first time, in exchange for Ukraine withdrawing from the parts of Donbas it still controlled. Though Trump then suggested that Russia and Ukraine could engage in some “swapping of territories”, Zelenskyy said he understood that Russia was “simply offering not to advance further, not to withdraw from anywhere” and that swaps were not on the table.

“We will not leave Donbas. We cannot do it,” Zelenskyy said. “For Russians, Donbas is a springboard for a future new offensive.” The region demanded by Russia was too strategically important to give up, he said, because it was a heavily fortified area that protected Ukraine’s central cities.

“I have heard nothing – not a single proposal – that would guarantee that a new war will not start tomorrow and that Putin will not try to occupy at least Dnipro, Zaporizhzhia and Kharkiv” once Russia had gained all of Donbas, Zelenskyy said.

Ukraine’s leader said he wanted Putin instead to agree to a ceasefire on the current frontlines and for both sides to return all prisoners of war and missing children, before any discussion about territory and the future security of the country. “Any question of territory cannot be separated from security guarantees,” he said.

Zelenskyy said he would not be at the summit in Alaska, the first face-to-face meeting between Trump and Putin with both in office since 2018. But he said he hoped it would be followed by “a trilateral meeting” with Trump and Putin, though the Russian leader has so far said he is not willing to meet Zelenskyy.

The Ukrainian leader also expressed faith in the unpredictable Trump, who he said could act as an honest broker between himself and Putin. “I do not believe that Putin’s proposal is Trump’s proposal,” he said. “I believe that Trump represents the United States of America. He is acting as a mediator – he is in the middle, not on Russia’s side. Let him not be on our side but in the middle.”

He said he did not know what exactly Putin and Trump were going to discuss in Alaska, saying “probably there is a bilateral track” of talks about other topics of mutual interest, such as trade, sanctions and business. But he said Putin had scored a diplomatic win in securing the meeting: “He is seeking, excuse me, photographs. He needs a photo of his meeting with President Trump.”

Zelenskyy said Russia was desperately trying to show it was winning the war and that the Kremlin wanted “to create a certain narrative, especially in the American media, that Russia is moving forward and Ukraine is losing” by mounting sabotage attacks in the Donbas region.

He acknowledged that “groups of Russians advanced about 10 kilometres in several places” although he said: “They have no equipment, only weapons in their hands,” and said that some had already been killed or captured.

But the breach is ill-timed from Ukraine’s point of view. In Alaska, Putin is likely to tell Trump that such successes show that Russia is gradually winning the three-year war in the east, and so US future support for Kyiv will be wasted.

War maps showed two lines of advance east of the town of Dobropillya, and gains of about six miles since Friday. Experts said the next few days would be critical to see if Ukraine could contain the break in the front.

Ukraine’s military said Russia had concentrated about 110,000 troops in the sector and that the invaders were “brazenly attempting to infiltrate our defensive lines with sabotage and small infantry groups, regardless of their losses”.

The military command said in a social media post that reserves had been deployed at the order of Gen Oleksandr Syrskyi, Ukraine’s chief military commander, in an effort to restore the frontlines.

The Institute for the Study of War said Russian “sabotage and reconnaissance groups” had infiltrated Ukrainian-held territory near Dobropillya, a key supply point in the west of the Donetsk region.

“It is premature to call the Russian advances in the Dobropillya area an operational-level breakthrough,” the ISW said on Monday night. It said the invaders would now try to turn “tactical advances” into something more significant.

Russia is taking heavy casualties of about 1,000 a day, with 500 killed and 500 wounded on Monday, Zelenskyy said, as it relies heavily on infantry assaults to break Kyiv’s defensive lines.

Zelenskyy said Ukraine’s casualties on the same day were much smaller – a total of 340 – “18 killed and 243 wounded, with 79 missing in action”. But in the past when Moscow’s forces have broken through, Ukraine has frequently proved unable to push them back.

A former senior Ukrainian army officer, Bohdan Krotevych, said the piercing of Ukraine’s lines had come about because “instead of reinforcing defensive units with infantry”, senior commanders in Kyiv had prioritised deploying newly mobilised soldiers into assault forces, leaving units already on the frontline weakened.

“To stabilise the front, we must reinforce brigades on the line of contact with infantry,” Krotevych said, and he called for Ukraine to urgently strengthen its reserve forces and adopt a defensive strategy rather than try to counter high-risk Russian infantry assaults with its own.

Dobropillya is a key supply point for the beleaguered towns of Pokrovsk and Myrnohrad to the south and the principal cities of Ukrainian-held Donbas to the east from the centre of the country.

Zelenskyy said Russia was preparing a fresh offensive in the autumn involving nearly 30,000 troops moved from Sumy, in the north-east of Ukraine, “in three directions” on the frontline – towards Zaporizhzhia in the south and Pokrovsk and the nearby Novopavlika in the south-east.