Kremlin says aid caravan will be under aegis of Red Cross and will depart this week, as US and Kiev plan separate aid mission

Russia is sending a humanitarian convoy into Ukraine in cooperation with the Red Cross, the Kremlin has announced.

Western leaders had previously warned Vladimir Putin, the Russian president, against using a humanitarian aid mission as a pretext for a military intervention in the conflict in eastern Ukraine. But Putin's spokesman, Dmitry Peskov, said the convoy would not include any armed personnel or soldiers.

"It is being sent under the aegis of the Red Cross and with the agreement of Kiev," Peskov said, adding that the convoy would probably depart this week.

In a telephone call with his Ukrainian counterpart, Petro Poroshenko, on Monday night, Barack Obama promised to take an active part in an aid mission to the rebel-held city of Luhansk, where residents have been cut off from water and electricity amid shelling that has claimed civilian casualties.

The Ukrainian presidential spokesman, Svyatoslav Tsegolko, said: "The president of the United States expressed his support for the president of Ukraine's actions and in particular his initiative for an international humanitarian mission to Luhansk under the aegis of the International Committee of the Red Cross, with the participation of the European Union, Russia, Germany and other partners."

The ICRC said it had met with Ukrainian and Russian authorities and discussed how a possible humanitarian aid mission could take place without an armed escort.

The EU commission president José Manuel Barroso warned the Kremlin 'against any unilateral military actions in Ukraine, under any pretext'. Photograph: Olivier Hoslet/EPA

"The practical details of this operation need to be clarified before this initiative can move forward," said Laurent Corbaz, the ICRC's head of operations for Europe and Central Asia.

Ukraine had previously objected to Russia sending any aid to the region, and the west has strongly warned Russia that any attempt to send its military personnel into Ukraine under the guise of humanitarian assistance would be seen as an invasion.

The European commission president, José Manuel Barroso, cautioned Putin against military intervention in Ukraine during a phone call on Monday. "President Barroso warned against any unilateral military actions in Ukraine, under any pretext, including humanitarian," the commission said in a statement.

Earlier, the secretary general of Nato, Anders Fogh Rasmussen, compounded international tension over a potential Russian invasion of Ukraine, telling Reuters there was a "high probability" of a Russian military intervention.

"We see the Russians developing the narrative and the pretext for such an operation under the guise of a humanitarian operation and we see a military buildup that could be used to conduct such illegal military operations in Ukraine," he said.

A spokesman for the Ukrainian military, Andriy Lysenko, said Russia had 45,000 troops on its shared border, backed up by tanks, missile systems, warplanes and attack helicopters. Nato officials said last week that 20,000 Russian soldiers were at the border.

A Ukrainian tank near Donetsk, where rebels said they may be ready for a ceasefire. Photograph: Roman Pilipey/EPA

The Kremlin move comes after Ukraine government forces turned down a ceasefire offer and redoubled their offensive against pro-Russian rebels in eastern Ukraine despite the threat of a Russian military intervention.

Military troops and volunteer battalions had launched a four-pronged attack against rebel-held areas, national security council spokesman Lysenko said, cutting the link between the rebels' two main strongholds, Donetsk and Luhansk. He said Kiev forces were now "preparing for the final stage in the liberation of Donetsk".

Ukrainian government forces were also attacking the Alchevsk-Stakhanov area and the corridor between Snezhnoye and Torez, near where Malaysia Airlines flight MH17 went down, and have besieged the strategically placed city of Krasny Luch. Lysenko reported 26 military engagements between the two sides over the past 24 hours and said six Ukrainian soldiers had been killed.

In a sign that their backs may be to the wall, the Donetsk rebels have changed leadership and said on Saturday they were ready for a ceasefire. Kiev rebuffed a truce offered by commander Alexander Zakharchenko, who replaced Russian citizen Alexander Borodai as PM of the self-declared Donetsk People's Republic last week, saying the rebels must first surrender their arms.

Ukrainian forces have bombarded Donetsk in recent weeks, and heavy shelling was reported across the city on Sunday, with periodic volleys continuing overnight and on Monday.

Five shells landed at a high-security prison colony, killing one convict and setting off a mass jailbreak that allowed a reported 106 inmates to flee. According to a statement by Ukraine's penitentiary service, 34 of the escaped convicts had returned by Monday afternoon. Many of the others "had notified the institution's management by phone that they intended to return to the colony to continue serving their sentences as soon as the situation stabilises".

The high council of the self-declared Donetsk People's Republic previously said it would consider convicts' requests to atone for their crimes by serving in the rebel ranks.

Lysenko blamed the rebels for shelling the prison colony, as well as residential areas in Donetsk and Luhansk, claiming that Russia was ordering the bombardment to justify the deployment of Russian peacekeeping troops. While both sides are known to have heavy artillery, including notoriously imprecise Grad rocket launchers, Human Rights Watch has said Ukrainian Grad strikes had killed civilians on the Donetsk outskirts.

Lysenko said 568 Ukrainian troops had been killed in the four months of fighting in eastern Ukraine, and 2,120 wounded. One of those killed on Sunday was volunteer battalion fighter Nikolai Beryozovoi, the husband of well-known journalist Tetyana Chernovil, whose beating in December became a symbol of the brutality of the Viktor Yanukovich regime.

A UN report at the end of July said 1,129 civilians had been killed and 3,442 injured in the conflict.

The border service said Russian forces continue to shell Ukrainian territory, hitting two border posts overnight with artillery and mortar fire. It also accused Russia of violating Ukraine's airspace with swarms of surveillance drones, reporting five such flights overnight.

Photographs published on Sunday by anti-terrorist officials purported to show the aftermath of a Grad attack from Russian territory that destroyed the village of Stepanovka near the border. "This once flourishing settlement where more than 1,000 people lived, where children's laughter could be heard, where weddings were celebrated, has now been completely destroyed by BM-21 Grad volleys," the press servicea spokesperson said.

The Interfax news agency quoted a source in Brussels saying that the EU was pushing for international organisations to oversee the delivery of humanitarian aid to eastern Ukraine and for refugee corridors to be created. Russia's foreign minister, Sergei Lavrov, said on Sunday that Moscow was negotiating with Kiev to deliver humanitarian aid to eastern Ukraine. Ukraine's president, Petro Poroshenko, told the US secretary of state, John Kerry, on Sunday night that he had invited the Red Cross to facilitate an international aid mission.

Water, electricity and phone lines have been cut off in the besieged city of Luhansk for more than a week and 5,000 people were without electricity in Donetsk on Monday, according to local authorities.

The EU source also reportedly said Barroso would speak with Poroshenko about a ceasefire. But Lysenko said Kiev's "anti-terrorist operation" in the east would not be stopped and that corridors already existed for civilians to flee the conflict zone.