Fighting was heaviest around Handarat, a hilly area that lies along a route leading north out of opposition-held parts of the city.
Observatory head Rami Abdel Rahman said regime forces, backed by Russian and Syrian planes, were seeking to cut off that road and "completely besiege" eastern neighbourhoods.
An AFP correspondent in opposition-held eastern Aleppo city said the booming crashes of strikes could be heard throughout the day, but no raids hit the city itself.
"We're overwhelmed once again by fears of a government siege of Aleppo, after weeks of a ceasefire that may have just collapsed," said Ali Saber, a 32-year-old father of one living in Aleppo.
The assault has sparked "strong concerns" in Washington about the ceasefire, which saw violence plummet after it took effect on February 27.
A senior US administration official said the offensive "could well violate the cessation of hostilities, which has... been under increasing pressure in recent weeks".
European Union foreign policy chief Federica Mogherini also expressed concern about the Aleppo offensive and called for the "respect, consolidation, and expansion" of the truce.
Representatives of Bashar al-Assad's regime are expected in Geneva on Friday for peace talks with UN envoy Staffan de Mistura, who criticised the government for hindering efforts to deliver life-saving assistance to Syrians in need.
- 'Disappointing' aid situation -
President Vladimir Putin, Assad's main backer, said during an annual call-in show on Russian TVB on Thursday that he was "closely following" the latest spike in violence.
Putin last month announced a partial pullout from Syria, but warned Russia could return in full force should the situation require it.
The UN fears that the humanitarian situation may deteriorate once again after reporting significant progress through the first two weeks of March.
"We are extremely concerned about the situation in northern Syria, including Aleppo governorate, where there has been a significant increase in incidents of violence that continue to aggravate the humanitarian situation," a UN humanitarian official told AFP.
The official said the UN was planning several aid deliveries to Afrin and Azaz -- two flashpoint areas in the province -- in the coming days.
But UN envoy de Mistura said restrictions by Damascus have frustrated efforts to deliver much-needed assistance to besieged Syrians.
Despite several UN Security Council resolutions that call for unobstructed aid access to besieged areas, the regime continues to frequently deny passage to convoys carrying assistance.
"Everyone in the meeting was disappointed," de Mistura told reporters after a weekly meeting of the so-called humanitarian taskforce on Thursday.
"Many of (the countries) are actually frustrated by the lack of new convoys," he added, naming Douma and Daraya, both besieged by the regime near Damascus, as being in dire need of aid.
De Mistura said he made some progress in Damascus by securing tentative permission to distribute medical supplies.
But he said that the government had specifically excluded surgical equipment, anti-anxiety pills and atropine, which can be used to guard against poison -- including sarin gas.
- IS seizes villages -
Washington expressed outrage over the death of a doctor who ran a hospital and field clinics in the northern town of Hama, which is held by anti-regime rebels, saying it appeared he had been deliberately targeted in an area where Russian planes operate.
Syria's conflict began in 2011 as a peaceful revolt, with protests across the country that spread in 2012 to Aleppo province, which borders Turkey.
Rebel groups, some of them allied with Al-Qaeda's Syrian affiliate, Al-Nusra Front, are fighting the government, rival extremists and Kurdish militia there.
The alliance between opposition fighters and Al-Nusra has complicated the implementation of Syria's truce, which excludes Al-Nusra and the Islamic State group.
The Observatory said Thursday IS fighters had seized a string of opposition-held villages near Syria's border with Turkey.
The most important among them was Hiwar Kallis, about one kilometre (less than one mile) south of the Turkish frontier.
Rebels had secured significant gains against IS in the area before the jihadists pushed them back this week.