DONETSK, Ukraine -- Shells smashed into a residential neighborhood of Donetsk on Tuesday as Ukrainian forces intensified their campaign to encircle the rebel stronghold. The shelling killed at least two people, blew gaping holes in an apartment block and raised fears that the city is on the verge of severe bloodshed.

Fighting also raged elsewhere in Ukraine's troubled east, bringing the death toll to at least 24 civilians and 10 soldiers over the past day. And it prevented international investigators once again from visiting the site of the Malaysia Airlines jet shot down earlier this month.

The increased danger to civilians has brought sharp criticism from the United Nations and human-rights groups. But each side blames the other for shelling residential areas.

The rebels insist the attacks are evidence of what they describe as the government's indiscriminate oppression of its own people. But Ukraine insists it has banned the use of artillery in residential areas and in turn accuses separatists of targeting civilians in an effort to discredit the army.

"This is done by terrorists," said Andriy Lysenko, a spokesman for Ukraine's national security council, referring to the shelling of civilians. "Under instructions from the president, in residential areas and communities where we have Ukrainian citizens, we do not fire artillery or perform airstrikes."

Donetsk until recently had seen little fighting other than a rebel attempt in May to seize the city's airport. But Tuesday's barrage, along with last week's shelling of the city's main railroad station, has brought the war painfully close to the city of nearly 1 million. Ukrainian forces have made advances against rebels in nearby towns.

Fighting between Ukrainian forces and pro-Russia separatists also has been heavy around Luhansk, the second-largest city held by the rebels. Five people died when artillery fire hit a home for the elderly there Monday, local authorities said.

The Donetsk shelling sent about 50 frightened residents to huddle for safety in an underground parking lot, including Lubov Skorikh who was distraught at discovering that her husband Vladimir had been killed.

"I ran out. ... An old woman told me, 'Look, there is a man lying there.' I didn't even think that could be my husband. But then I saw the shoes; they were his shoes," she said, breaking down in tears. "Do you understand? His shoes! My God, I lived with him for 45 years; he was such a good person."

On Tuesday, the national rail system said it would offer free transport to people leaving the areas engulfed in fighting. But Lysenko, the security council spokesman, said rebels had blocked the railroad out of Luhansk, barring residents from leaving the city.

Lysenko also accused separatist fighters of using children as human shields and stopping cars from leaving Luhansk. It was not immediately possible to confirm those claims.

In Horlivka, a city besieged by government troops, the mayor's office reported Tuesday that 17 people, including three children, were killed as a result of shelling.

A United Nations monitoring mission in Ukraine says there has been an alarming buildup of heavy weaponry in civilian areas of Donetsk and Luhansk -- including artillery, tanks, rockets and missiles that are being used to inflict increasing casualties and damage to civilian infrastructure.

The U.N. said in a report this week that use of such weaponry could amount to a violation of international humanitarian law.

The U.N. report acknowledged the government's promise not to bombard the cities of Donetsk and Luhansk. However, it said that "people trapped in areas controlled by the armed groups continue to be killed as the heavy shelling continues from both sides."

The U.N. called for a "full and impartial investigation" of all strikes where civilians have been killed.

The overall death toll has been steadily rising. The U.N. has said that at least 1,129 people were killed between mid-April, when fighting began, and Saturday.

Information for this article was contributed by Jim Heintz of The Associated Press.

A Section on 07/30/2014