— After several days of setbacks, Ukrainian security forces sought to take the offensive Tuesday from pro-Russian insurgents in the country’s southeast, launching a major assault on a rebel stronghold.

The attack on separatist positions in the city of Slovyansk began at dawn and continued throughout the day, with officials and residents reporting fierce clashes.

Much of the fighting, however, was still conducted from a distance, with Ukrainian forces pounding rebel positions with artillery fire, residents said. Although troops were massing around Slovyansk, it was unclear whether they would attempt to enter the heart of the city — a move that could lead to high casualties on both sides and among civilians. Residents were urged Tuesday to remain inside their homes while battles raged on the outskirts of Slovyansk.

At least 181 people, including 59 Ukrainian soldiers, have died in clashes in the east since fighting began in April, according to Ukraine’s top prosecutor, Oleh Makhnitsky. Ukrainian officials say they do not keep track of insurgent deaths, so the actual total is likely to be far higher.

Many in the southeast fear that the number could rise dramatically if the military launches a campaign to retake the many towns and cities across the region that have been commandeered by insurgents.

Slovyansk is widely considered one of the rebels’ best-fortified positions, and the insurgent leaders in the city have aggressively deployed their forces to take down government aircraft and ambush troop convoys.

Stella Khorosheva, a spokeswoman for the rebels in Slovyansk, called Tuesday’s assault “the heaviest and longest attack” since the government’s counterinsurgency operation began several weeks ago.

Khorosheva said the city came under bombardment from artillery and helicopter gunships at 6 a.m., with only brief pauses throughout the day. Ukrainian troops, she said, were massing on the city’s outskirts and appeared poised to move in.

“They’re going to destroy the whole city,” she said.

Writing on his Facebook page, Ukrainian Interior Minister Arsen Avakov said government troops had broken through rebel checkpoints on the outskirts of the city in what he described as “a very intense firefight.”

Vladislav Seleznev, a Ukrainian army spokesman, had said earlier that a convoy of troops in armored personnel carriers was ambushed overnight while en route to Slovyansk, leaving one soldier dead and 13 wounded.

The violence in southeastern Ukraine has intensified in recent weeks, particularly after the election of chocolate tycoon Petro Poroshenko as the country’s new president. The vote largely did not take place in the southeast because conditions were too volatile. Poroshenko is due to be sworn in Saturday.

About 350 recruits are engaged in exercises with Ukraine's national guard in preparation for deployment to the east. (Reuters)

The new president’s first order of business will be to decide how aggressively to take on the insurgency, which has appeared to gain traction as reinforcements and supplies have flowed in across a porous border with Russia.

On Monday, guards fended off a coordinated assault by up to 500 rebels on a border command center in the far-eastern region of Luhansk. Oleg Slobodyan, a spokesman for the Ukrainian border service, said that the area was quiet Tuesday but that there were indications that insurgents were regrouping for another attempt.

In a sign of the worsening conditions across the southeast, the Organization for Security and Cooperation in Europe said it would curtail its monitoring work in the Donetsk and Luhansk regions, both of which have declared themselves to be “sovereign” republics.

The OSCE has lost contact with two groups of international monitors in the past week, with both presumed to have been detained. Spokesman Michael Bociurkiw said the OSCE was asking anyone with information about the monitors to contact the organization.

Morello reported from Kiev. Daniela Deane in London contributed to this report.