Syrian regime forces regained ground Tuesday in Aleppo, threatening to cut off a lifeline that rebels forced through into the city's besieged eastern sector, a monitoring group said.
Government troops and allied militia fighters regained firing positions over the route through the city's Ramousseh district, the Syrian Observatory for Human Rights said.
The government's military press office said that "the breach the gunmen made on the Ramousseh axis towards eastern Aleppo has been completely closed."
Observatory director Rami Abdel Rahman said the rebel lifeline had not been recaptured by the regime, but government forces and allied militias had advanced with the support of intense air raids, and could now fire on the route.
The advance came a day after the Syrian government and its ally, Lebanese Shiite militia Hezbollah, sent thousands of reinforcements to mount a counterattack in Aleppo after rebels broke through government lines two days earlier.
Rebels had vowed that a "new stage" in their offensive would aim to "liberate all of Aleppo," after they fought their way through Saturday to the besieged opposition-held eastern sector of the city.
Syrian state news agency SANA posted videos of government planes carrying out what it said were "intensive strikes" on "terrorist movements" south of Aleppo.
Syria's largest city before the war, Aleppo has been divided between government forces in the west and rebels in the east since fighting for control of the city erupted in mid-2012.
Last month government forces captured the last remaining rebel supply route into the east, known as Castello Road, raising fears that some 250,000 to 300,000 civilians there would come under siege.
The United Nations warned that food supplies in the east would last only until mid-August. On Tuesday, after the rebel breakthrough cut off the main government supply line to the west, UN children's agency UNICEF said about 2 million people on both sides of the city had no access to drinking water.
At a meeting of the UN Security Council, UN humanitarian affairs chief Stephen O'Brien reiterated his call for regular 48-hour humanitarian pauses in Aleppo, noting that "very brave" aid workers on the ground have been unable to access the city because of the security situation.
"They are brave, but they are not suicidal - we have to have security conditions in place," O'Brien said.
Vitaly Churkin, Russian ambassador to the UN, told reporters that Russia and the US were discussing the details of a plan to allow delivery of aid through Castello Road "on a permanent basis."
Samantha Power, US ambassador to the UN, warned that the risks faced by people in eastern Aleppo were "grotesque" and the situation was deteriorating in the city's western area, too.
She said that the peace talks needed to get back on track urgently: "The political stakes could not be higher."
In Washington, US State Department spokeswoman Elizabeth Trudeau called the Aleppo situation "dire."
"The re-institution of a cessation of hostilities ... is the only path forward," Trudeau said.
Meanwhile, in north-eastern Syria, at least 15 people including three children, were killed in airstrikes on a village near the eastern city of Deir al-Zour, the Observatory said.
Doctor Mohammed Bashir from Mahamadiyeh village told dpa that 21 people were killed and more than 45 were wounded in the strikes.
Deir al-Zour province is controlled by Islamic State, apart from a regime-held pocket of Deir al-Zour city.
US-backed, Kurdish-led forces on Tuesday said they had captured the centre of Minbij city in northern Syria from Islamic State.
The jihadists were still holed up in the city's northern district and were holding an estimated 4,000 to 5,000 civilians there as "human shields," Shervan Darwish, spokesman for the Minbij Military Council, told dpa.
The Kurdish-led forces began their campaign to capture Minbij at the end of May, backed by US airstrikes.
They aim to cut Islamic State off from its last remaining route to the Turkish border and the outside world, preventing it from bringing supplies and fighters in to the rest of its territories in Syria and Iraq.