Shelling resounded on the outskirts of a key Ukrainian port as Russian-backed rebels pressed their south-east offensive, just hours ahead of talks that are widely hoped to bring a ceasefire.
Mariupol lies along the Sea of Azov, between Russia to the east and the Crimean Peninsula to the west, which Russia annexed in March.
The sound of incoming and outgoing shelling from different directions appeared to indicate that rebels have partially surrounded the area and are probing its defences.
The onslaught could be aimed at increasing pressure on the Ukrainian government ahead of peace talks in Minsk, Belarus.
The seizure of Mariupol would give the rebels a strong foothold on the Sea of Azov and raise the threat that they carve out a land corridor between Russia and Crimea.
If that happens, Ukraine would lose another huge chunk of its sea coast and access to the rich hydrocarbon resources the Sea of Azov is believed to hold.
Ukraine has already lost about half its coastline, several major ports and untold billions in Black Sea mineral rights with Russia's annexation of Crimea.
"Mariupol is a strategic point. If we lose it then we could lose the entire coastline, the whole south of Ukraine," said Tatyana Chronovil, a prominent Ukrainian activist at a mustering point for the volunteer Azov Battalion on the eastern edge of the city.
The rebel offensive follows two weeks of gains that have turned the tide of the war against Ukrainian forces, who until recently had appeared close to crushing the five-month rebellion in the east.
Ukraine and the West say the rebel counter-attack was spearheaded by regular Russian army units, a charge the Kremlin has denied.
Representatives of Ukraine, Russia, the rebels and the Organisation for Security and Cooperation in Europe are to meet later in Minsk for talks on ending the conflict.
President Petro Poroshenko said at a Nato summit in Wales that he was ready to order a ceasefire in eastern Ukraine if a deal is signed. The rebels said they were ready to declare a truce if agreement can be reached on a political settlement for the mostly Russian-speaking region.
Saying he is "ready to do my best to stop the war," Mr Poroshenko voiced "careful optimism" about Friday's meeting. Earlier this week, he discussed the outlines of a peace deal with Russian President Vladimir Putin, who also expressed optimism about the chances of reaching an agreement.
For all the upbeat assessments, however, Nato Secretary-General Anders Fogh Rasmussen was still sceptical of Russian motives.
"What counts is what is actually happening on the ground," he said at the Nato summit. "I have to say that previously we have seen similar statements and initiatives and they have been a smokescreen for continued Russian destabilisation of the situation in Ukraine."
Since mid-April, Moscow-backed separatists have been fighting government troops in a conflict the UN estimates has killed nearly 2,600 people. On Thursday, a Nato military officer said the ranks of Russian soldiers directly involved in the conflict have grown.
Colonel Andriy Lysenko, a spokesman for Ukraine's national security council, said seven servicemen had been killed over the past day, bringing the Ukrainian forces' death toll to 846.