MOSCOW--Ukraine suffered heavy losses Thursday in a counterattack along a key supply route to the separatist capital of Donetsk, signaling that despite steady advances by government forces pro-Russian rebel fighters still pose a significant threat.

Losses have mounted recently and the fierce fighting adds more pressure on Kiev to seek a compromise at peace talks next week in the Belarusian capital of Minsk, where Ukrainian President Petro Poroshenko and Russian leader Vladimir Putin are scheduled to meet face-to-face for the first time in two months.

The attack in the town of Ilovaisk left 19 fighters from pro-Kiev volunteer battalions dead and 42 wounded, said senior Interior Ministry aide Anton Gerashchenko, without giving a time frame. The government has been steadily eating away at rebel-controlled territory and had said it regained control of Ilovaisk on Wednesday.

Col. Andriy Lysenko, spokesman for Ukraine's National Security and Defense Council, said the government had repulsed the insurgent counterattacks and that reinforcements had been sent to help bolster the town's defenses. Separatist leaders, however, said they had surrounded Ukrainian units in the town and prevented reinforcements from getting there.

The losses in Ilovaisk, "shows how fierce the fighting is there," Mr. Gerashchenko wrote in a post on his Facebook page. The four battalions fighting in Ilovaisk are from four volunteer units, and Mr. Gerashchenko said they were acting effectively as army units but without heavy weaponry.

Kiev forces have increasingly relied on the committed but poorly trained volunteer units in recent weeks. Mr. Gerashchenko wrote in his post Thursday that a problem in the operation was a lack of coordination with the army, which provides artillery support for operations.

The monthslong conflict has begun to weigh heavily on Ukraine's already weakened economy. On Thursday, Economy Minister Pavlo Sheremeta said he had submitted his resignation because he was unhappy with the pace of reforms and the appointment of a trade representative without his approval.

Mr. Sheremeta's departed with a broadside at the government's attempts to overhaul its moribund economy, which is forecast to contract by more than 6% this year.

"We need to change this whole paradigm: The system, the people, the processes," Mr. Sheremeta said Thursday.

Prime Minister Arseniy Yatsenyuk, an economist who threatened to quit last month over the slow pace of reform, has warned that restoring infrastructure in eastern Ukraine could cost billions of dollars and that the conflict was hamstringing much-needed economic overhauls.

At the same time, Col. Lysenko pointed to government progress in the battle for control of the rebel's easternmost stronghold of Luhansk. Over the weekend, Ukraine took control of a district police station not far from the city and on Wednesday, Col. Lysenko claimed troops had taken control of "a significant part" of the city," without specifying what additional gains had been made.

The rebels deny that the government has made any serious advances into the city. On Thursday, Col. Lysenko said that fierce fighting continues to take place there and that soldiers working in small mobile groups were moving to take out rebel units and checkpoints.

Meanwhile, Ukrainian authorities issued a directive that a convoy of nearly 300 trucks sent by Moscow carrying what the Russians say is humanitarian aid be inspected before crossing into Ukraine.

Col. Lysenko said Ukrainian officials working on the Russian side of the border were waiting for final confirmation from the International Committee of the Red Cross that it had received security guarantees from the rebels and was ready to deliver the cargo.

The trucks have remained in a field near the border since for a week. Sixteen of the trucks were moved to the Russian border point at the weekend, and more began to head out in that direction from the field Thursday.

Write to James Marson at james.marson@wsj.com

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