The death toll from conflict in eastern Ukraine has doubled in the past fortnight, the UN's human rights office said on Wednesday, as international wrangling continued over a controversial Russian aid convoy to the region.

The UN office said its "very conservative estimates" suggested the death toll has risen to 2,086 by the beginning of this week, up from 1129 on 26 July. About 5,000 people had been injured, it said, in figures that represented "a clear escalating trend" of violence in the east.

The Ukrainian army and Kiev-backed volunteer battalions have been on the offensive against pro-Russia rebels in the east, pushing them back into the regional capitals of Donetsk and Luhansk and clearing them from smaller towns en route. But the tactics from both sides have been controversial, with shelling of residential areas leading to daily civilian casualties. The UN figures included civilians, Ukrainian soldiers and armed rebels.

There were reports on Wednesday that Igor Strelkov, the self-proclaimed defence minister of the rebel Donetsk People's Republic, had been badly wounded in fighting. Strelkov, a Russian citizen, has led the separatists' military operation against Ukrainian government troops. Ukraine says Strelkov is a senior officer with the Kremlin's GRU military spy agency.

Poland's foreign minister Radoslaw Sikorkski told Reuters on Wednesday he believed the Kremlin might still invade eastern Ukraine, though the probability of Russian troops entering with a convoy of supply trucks had receded somewhat.

The huge convoy set off from the Moscow region on Tuesday. The Kremlin says its 260 military trucks – hastily repainted white by Russian soldiers – contain humanitarian supplies for residents in the east trapped by fighting.

But the Kremlin's murky unilateral operation has raised fears that the mission's real goal is to smuggle fuel and ammunition to the rebels, or to spearhead a full-blown invasion.

On Monday, Nato secretary general Anders Rasmussen said there was a "high probability" of Russian attack.

On Wednesday night, the convoy appeared to have ground to a halt in the city of Voronezh, several hours' drive from Ukraine. It had been due to leave Voronezh on Wednesday morning but remained there late in the evening. Journalists tweeted photos of dozens of white vehicles parked inside a military base.

Arsen Avakov, Ukraine's interior minister described Moscow's convoy as "a provocation by a cynical aggressor". Several top officials said the trucks would not be allowed on Ukrainian soil, despite earlier suggestions that some kind of agreement had been reached. Prime minister Arseny Yatseniuk was scathing: "First they send tanks, Grad missiles and bandits who fire on Ukrainians. Then they send water and salt."

Kiev accuses Moscow of fuelling the conflict in the east by supplying arms and mercenaries – a charge the Kremlin denies. Yatseniuk said Moscow should instead "send 300 empty trucks and take their terrorists back", and then there would be no need for humanitarian help.

A spokesperson for President Petro Poroshenko did suggest late on Wednesday that there was a possibility that the convoy could cross into Ukraine closer to Luhansk and be cleared through customs by Ukrainian officials jointly with representatives of the Organisation for Security and Cooperation in Europe (OSCE). But it was unclear where exactly the trucks were heading. The separatists control about 60 miles of the border with Russia. The Izvarino border crossing near Luhansk is effectively beyond the control of the Ukrainian border troops.

The International Committee of the Red Cross (ICRC) - which was told by Moscow last week of a possibly shipment - said it was still awaiting details of where the convoy was heading, and what would happen when it got there. "We are missing several pieces of the puzzle," ICRC spokesperson Ewan Watson told the Guardian. He added: "This isn't our convoy". Viktoria Zotikova, an ICRC representative in Moscow, added: "We are willing to take over the assistance from Russia and be in

charge of the convoy, but we can't without the Ukrainian authorities. What's important is that they decide how it will cross, how it will go further, how the supplies will be stored. We would have to know for sure that the Ukrainian side has checked the cargo and agreed on the contents of it."

Meanwhile, Russian President Vladimir Putin arrived in Crimea as part of a two-day visit to the Black Sea peninsula which he annexed from Ukraine in March. Putin met with Russia's top intelligence chiefs to discuss local security issues. He is due to convene the top echelons of Russia's government in the port of Sevastopol on Thursday, with dozens of top officials flying in from Moscow in a clear signal to the west that Crimea is Russian territory.

Russia has been under pressure from the west for its actions in Crimea and eastern Ukraine, with several rounds of sanctions imposed by the EU and the US, the most recent coming after the downing of Malaysian airliner MH17 over eastern Ukraine, apparently with a missile fired by the separatists Russia backs. For months, Russian forces have been built up along the border with Ukraine.