The Syrian government has sent thousands of reinforcements to mount a counterattack in Aleppo after rebels broke through government lines two days earlier, a monitoring group said.
The Lebanese Shia Hezbollah movement and the government have mobilised more than 3,000 troops and militia fighters for an attempt to recapture the areas where the rebels made their breakthrough, according to the Britain-based Syrian Observatory for Human Rights (SOHR).
"Hundreds of opposition fighters have also arrived in Aleppo from the [neighbouring] province of Idlib to help in the expected battles," said Rami Abdel Rahman, head of the group.
Hezbollah's battle-hardened fighters have provided crucial support to Syrian President Bashar al-Assad's overstretched and exhausted army.
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Syrian state news agency SANA said government planes carried out "intensive strikes" on what it called "terrorist movements" south of Aleppo.
The government also acted to quell fears that rebel advances would cut supply lines to government-held western Aleppo. SANA quoted provincial governor Mohammed Marwan al-Oulabi as saying that all essential goods and fuel were still available in the area.
"Dozens of fuel tanks entered Aleppo city on Monday," he said.
The influx of pro-government troops comes as the rebel Army of Conquest coalition vowed that a "new stage" in its offensive would aim to " liberate all of Aleppo".
The group, a coalition that includes Jabhat Fateh al-Sham (formerly the al-Nusra Front), first broke a government siege on the rebel-held half of the city on Sunday, and has reportedly doubled its number of fighters in an effort to retake the entire city.
The UN on Tuesday called for an urgent humanitarian pause in fighting in Aleppo to allow access to repair electricity and water networks, and to provide aid. Millions could be affected if the pause were not put in place, it warned.
Children were being put at risk, UNICEF added, as water cuts in the province during a heatwave increase the chances of disease.
'About to escalate'
Once Syria's economic hub, Aleppo has been divided between government forces in the west and rebels in the east since fighting broke out for control of the city in mid-2012.
There are thought to be about 300,000 people living in the rebel-held east, and about 1.2 million in the government-controlled west.
Rebels say they helped to move people to safer areas during the battle. Humanitarian groups have warned, however, that there are no safe routes out of the city.
Al Jazeera's Reza Sayah, reporting from Gazientep on the Turkey-Syria border, said that both sides were ramping up the fight.
"There are increasing signs that the battle for Aleppo is about to escalate," Sayah said.
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The SOHR said on Monday that there were "continuous air strikes" on Aleppo and surrounding areas, with missiles "launched by the regime forces" killing at least two women, one of whom was pregnant.
SOHR says at least 290,000 people have been killed in the civil war, while the UN envoy for Syria, Staffan de Mistura, has estimated that some 400,000 people have been killed.
The battle comes as the United States and other world powers intensified a call on Russia to use its influence on Syria's government, urging Moscow at a special session of the United Nations Security Council on Monday to help to stop the bloodshed in Aleppo.
Samantha Power, US ambassador to the UN, asked Russia to "stop facilitating these sieges".
In response, Russia's deputy ambassador to the, UN Vladimir Safronkov, said: "I must say the propaganda and the emotional rhetoric, the unfounded accusations, the information campaign, means that we cannot move toward a political settlement in Syria."
Source: Al Jazeera and agencies