The Syrian Observatory for Human Rights reported shelling by both sides and "very intense clashes on every front line"
State media said rebel fire on government-held parts of the city had killed seven people, and a monitor said air strikes on remaining rebel-held territory had resumed.
An AFP correspondent in a rebel-held area of east Aleppo saw several wounded civilians nearby as the area came under heavy tank fire.
Terrified residents were running through the streets in a fruitless bid to find adequate shelter, he said.
The Syrian Observatory for Human Rights reported shelling by both sides and "very intense clashes on every front line".
"Regime forces have fired dozens of shells since this morning on areas held by the opposition factions, which have also fired at least eight shells so far into regime-controlled areas," Observatory head Rami Abdel Rahman said.
Six people were killed and others wounded in rebel fire on the government-held Bustan al-Qasr neighbourhood, state television said.
A seventh person was killed and more wounded in other government-held parts of the city, the television reported.
Government warplanes carried out strikes in several parts of the small pocket of territory still held by rebels in east Aleppo, the Observatory said.
The fighting erupted after several hours of calm following the announcement on Tuesday of a deal brokered by Russia and Turkey for the evacuation of civilians and rebel fighters.
The deal was to put an end to the army's month-long offensive on east Aleppo during which it has retaken more than 90 percent of the onetime rebel stronghold.
But rebels and a source close to the regime said the deal was on hold after objections from the Syrian government.
A source close to the government said Damascus objected to the number of people seeking to leave and wanted the names of evacuees.
"The government has suspended the evacuation deal because originally it was for the departure of 2,000 fighters and now there will be more than 10,000 people," he told AFP.
"The government also wants a list of the people who are leaving because they could include hostages or prisoners," he said.
Yasser al-Youssef, a political official from the Nureddin al-Zinki rebel group, accused the government and its ally Iran of "blocking civilians from leaving after adding new conditions" to the deal.