Fears are mounting in Ukraine of a possible Russian invasion in the guise of a “humanitarian” mission to the conflict-torn east where Kiev says another 13 of its troops have died in fighting with pro-Moscow rebels.

The United States has warned that any unilateral intervention by Russia, including in the form of a humanitarian mission, would be considered an invasion.

Shelling meanwhile continued in the rebels’ main eastern stronghold of Donetsk, which has become the new battleground in the four-month conflict between Ukrainian government forces and eastern separatists.

A journalist heard repeated shelling in the city throughout the morning. Local authorities said mortar fire hit neighbourhoods north and southwest of the centre, and one person was killed.

Ukraine’s forces, which have been battling to wrest control of eastern strongholds from the rebels, which the West believes to be backed by Russia, reported 13 casualties in the last 24 hours.

The Red Cross also announced it was stepping up its aid activities in Donetsk and the second main insurgent bastion of Lugansk where local authorities have warned of a looming humanitarian catastrophe with many residents left without water and power for days.

A top official from Ukrainian President Petro Poroshenko’s office revealed late Friday that Kiev had managed through diplomatic channels to halt a Russian convoy, including troops and military hardware, advancing towards the border under the guise of a humanitarian mission.

“The Ukrainian side has reasonable grounds to believe that the convoy could be used to further escalate tensions,” the foreign ministry added on Saturday.

The Red Cross acknowledged that it had received an offer from Moscow to organise aid convoys, and would welcome any effort to ease civilian suffering in the region, but denied Saturday that any such action had taken place.

NATO says Russia has 20,000 troops along the Ukrainian border, fuelling fears that Moscow could send them into its former Soviet neighbour.

US ambassador to the UN Samantha Power on Friday slammed Russian proposals to set up humanitarian corridors to east Ukraine.

A “unilateral intervention by Russia in Ukrainian territory, including one under the guise of providing humanitarian aid, would be completely unacceptable and deeply alarming and would be viewed as an invasion of Ukraine,” she said.

She drew a parallel with the 2008 crisis in South Ossetia, when Russia justified sending troops into the Georgian territory in response to civilian suffering.

Russia’s Deputy Foreign Minister Sergei Ryabkov hit back Saturday that Power’s comments demonstrated the extent of “anti-Russian hysteria” in Washington.

“Our proposal has clearly humanitarian objectives but our initiative is tossed aside and they only talk about how Russia would supposedly try to slip into Ukraine under the guise of humanitarian aid something that doesn’t please the US,” he told ITAR-TASS news agency.

In Donetsk, an AFP reporter heard loud explosions in two different locations around 6 am local time Saturday and again later in the morning. Apartment blocks and a hostel in the Kievsky district north of the city had shattered windows and there was shelling damage inside flats, AFP witnessed.

A local resident said: “It was the Ukrainian authorities. They have been firing on the (self-proclaimed) Donetsk People’s Republic all morning from the direction of the airport.”

The one-million-strong city has been at the heart of fighting in recent days as government forces seek to recapture rebel strongholds and cut them off from the Russian border. Almost 50 troops have been killed over the last three days alone.

So far, the four-month conflict has left over 1,300 dead and over 4,000 injured, according to the UN.