A burst of fighting in the flash-point Syrian city of Aleppo on Sunday appeared to shatter a unilateral Russian cease-fire, signaling a resumption of the government’s offensive to seize rebel-held areas there.

Warplanes — either operated by Syrian President Bashar al-

Assad’s government or by Russia, an ally — targeted the opposition neighborhoods of eastern Aleppo on Saturday evening, residents and rebels said. And, they said, pro-government fighters resumed shelling and ground attacks on rebel positions overnight Saturday that continued until Sunday afternoon.

“The attacks have started again, and they are so crazy, so intense,” said Zakaria Malahifji, a member of the Fastaqim rebel unit, which is battling in the northern city.

He added that the “rebels are preparing for a large-scale offense to break the siege of Aleppo.”

Pro-government forces have besieged rebel districts for weeks, leaving the more than 200,000 people in those areas in increasingly desperate circumstances.

[U.S. abandons efforts to work with Russia on Syria]

Moscow announced a “humanitarian pause” and halted air raids last week to allow rebels and civilians to flee Aleppo. Since 2012, the city has been divided between opposition districts to the east and government-

controlled neighborhoods to the west.

Since Saturday, rebel forces have warned residents in the east to avoid front-line areas, apparently in anticipation of more attacks, including those by pro-

government militiamen from countries such as Iran, Lebanon and Iraq.

Before the pause, Russian and Syrian government warplanes had been targeting homes, hospitals and infrastructure in the city’s east.

Russia recently signaled a major escalation in Syria, apparently dispatching warships, including an aircraft carrier that can hold as many as 40 planes. On Saturday, Dmitry Peskov, a spokesman for Russian President Vladimir Putin, upped the ante by vowing that all of Syria had to be “liberated” by Assad’s forces.

Aleppo has become a key battleground in the conflict, which began in 2011 and has killed more than 400,000 people. Seizing the eastern districts of Aleppo — ­Syria’s commercial hub before the war — would mark a major victory for Assad.

Fighting over the city, particularly pro-government attacks, has repeatedly torpedoed peacemaking efforts involving Washington and Moscow.

Forces aligned with Assad dramatically intensified attacks in recent weeks, throttling a cease-fire brokered for the city last month by Russia and the United States.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said Saturday that more than 2,100 civilians, including 479 children, have been killed in Aleppo in the past six months.

On Friday, the top U.N. human rights official, Zeid Ra’ad al-

Hussein, condemned the assaults on rebel areas of Aleppo as “crimes of historic proportions.”

Few, if any, of the estimated 200,000 residents in the war-

ravaged rebel neighborhoods took advantage of the Russian-led pause in fighting, choosing instead to stay put. U.N. officials attribute that to a lack of guarantees from the Assad government to help evacuate wounded and sick residents in opposition areas.

Government media accused rebel fighters — who rejected the cease-fire as “political theater” by Russia and the Syrian government — of preventing the sick and wounded from fleeing opposition areas. But residents of eastern Aleppo contacted by phone said people fear arrest and worse consequences if they leave.

They also have cited confusion about the parameters of the cease-fire. Text messages from the government said civilians had eight hours a day to leave rebel-held areas through designated corridors, but U.N. officials said Moscow informed them that it would last for 11.

Zakaria reported from Istanbul.

Read more:

Russian air defense raises stakes of U.S. confrontation in Syria

Incredible video shows Syrian boy dangling from wrecked building after airstrike in Aleppo

Today’s coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news