Almost 100,000 people remain trapped in the ruined city of Mariupol, facing starvation amid “constant” Russian bombardment, Ukraine’s President Volodymyr Zelenskiy has said, as he appealed for the release of a convoy he said had been captured by Russian forces.

In a video address late on Tuesday, he renewed his calls for Russia to allow safe humanitarian corridors, and said civilians faced “inhumane conditions. In a total siege. Without food, water, medication, under constant shelling and under constant bombing.”

More than 7,000 people had escaped the city in the past 24 hours, he said. However one humanitarian convoy travelling on an agreed route west of the city had been “captured by the occupiers”.

The convoy near Mangush consisted of bus drivers and emergency service personnel. “We are doing everything we can to free out people and unblock the movement of humanitarian aid.” he said.

Tens of thousands of residents have already fled the besieged southern port city, bringing harrowing testimony of a “freezing hellscape riddled with dead bodies and destroyed buildings”, according to Human Rights Watch.

Satellite images of Mariupol released by Maxar on Tuesday showed a charred landscape, with several buildings ablaze and smoke billowing from the city. The Pentagon has said Russia was now pummelling Mariupol using artillery, long-range missiles and naval ships deployed in the nearby Sea of Azov.

“They bombed us for the past 20 days,” said 39-year-old Viktoria Totsen, who fled into Poland. “During the last five days, the planes were flying over us every five seconds and dropped bombs everywhere on residential buildings, kindergartens, art schools, everywhere.”

“It is clear that the occupiers are not interested in the city of Mariupol, they want to raze it to the ground, to reduce it to ashes of a dead land,” one local official said.

Amid the bloodshed, Zelenskiy held out hope for negotiations, which have yielded little since the invasion began on 24 February. “It’s very difficult, sometimes confrontational,” he said. “But step by step we are moving forward.”

For the first time, there are signs that Ukrainian forces are going on the offensive, retaking a town near Kyiv and launching counter attacks in the south of the country, amid claims that Russian forces have just a few days of supplies remaining.

In the face of intense Ukrainian resistance, the Pentagon believes Russia’s forces may have been reduced by as much as 10% in the four weeks of fighting since the invasion began.

“The Russians may be slightly below a 90% level of assessed available combat power,” a senior defence official told reporters in Washington, adding that some Russian forces were suffering from frostbite.

The US president, Joe Biden, is due to travel to Brussels this week for Nato and G7 talks to discuss further action against Russia, including tighter sanctions. Biden discussed Moscow’s “brutal” tactics in a call with European leaders on Monday.

Biden will then head to Poland, which has received the bulk of more than 3.5 million Ukrainians fleeing war in their country. Zelenskiy will speak virtually at the Nato summit in Brussels on Thursday.

Amid concerns that Vladimir Putin, who Biden believes has “his back against the wall”, is considering the use of chemical and biological weapons, Kremlin spokesman Dmitry Peskov spoke to CNN about the circumstances in which Russia would use nuclear weapons.

Speaking to Christiane Amanpour on Tuesday, Peskov repeatedly refused to rule out that Russia would consider using nuclear weapons against what Moscow saw as an “existential threat.”

US Department of Defence spokesman John Kirby said Moscow’s nuclear remarks were “dangerous” and were “not the way a responsible nuclear power should act”.

However, Kirby added that Pentagon officials “haven’t seen anything that would lead us to conclude that we need to change our strategic deterrent posture”.

Peskov also insisted that the military operation was going “strictly in accordance with the plans and purposes that were established beforehand.”

In other developments:

• Russian forces “looted and destroyed” a laboratory at the Chernobyl nuclear power plant, Ukrainian officials said. “Russian occupiers illegally seized the newest laboratory,” Ukrainian authorities said in a statement late on Tuesday. The laboratory processes radioactive waste and contains “highly active samples” which are now in the hands of Russian forces, the agency added.

• The deputy head of Kyiv’s police force accused Russia of using white phosphorous munitions in the city of Kramatorsk in Donetsk. Oleksiy Biloshytskiy shared online footage, which could not be independently verified, of material burning fiercely underneath a pile of aggregate. “Another use of phosphorus ammunitions in Kramatorsk,” he said.

• The UN secretary general, Antonio Guterres, has said it is time for Russia to end its “absurd” and “unwinnable” war in Ukraine, as the EU prepared to set up a “trust fund” aimed at helping Kyiv repel the invasion and rebuild afterwards. “Even if Mariupol falls, Ukraine cannot be conquered city by city, street by street, house by house,” Guterres said.

• Russia plans to unleash a “great terror” on Kherson by kidnapping residents and taking them across the Russian border, an FSB whistleblower has claimed. The Kremlin was no longer willing to “play nicely” with protesters in the Ukrainian city, a letter said. About 300,000 people in the occupied southern city of Kherson are running out of food and medical supplies, a spokesperson for Ukraine’s foreign ministry said.

• Russian forces have “kidnapped” 2,389 children from the Russian-controlled areas of Luhansk and Donetsk, the US embassy in Kyiv has said, citing figures by Ukraine’s foreign ministry. The embassy said: “This is not assistance. It is kidnapping.”

With Agence France-Presse