Forces loyal to Syrian President Bashar al-Assad have cut off the only supply road to rebel-held areas in the northern city of Aleppo, state media and monitors reported on Thursday.
Citing an unnamed security official, Syria's state news agency SANA said the strategic Castello Road on the northern outskirts of Aleppo is under government firepower after they took full control over the southern part of the Mallah Farms area that overlooks the road.
The agency cited unspecified casualties among the "terrorists," a term it uses to refer to the rebels fighting to oust al-Assad.
The Syrian Observatory for Human Rights monitoring group reported fierce fighting in the area between regime forces and rebels, including the al-Qaeda-linked al-Nusra Front.
Unidentified warplanes carried out dozens of airstrikes in areas adjacent to Mallah Farms and Castello Road, according to the watchdog. No specific casualty numbers were given.
Military reinforcements were on their way to the rebels in the area, an Islamist rebel going by the nom de guerre Abu Rami told dpa by phone without giving details.
Earlier on Thursday, the observatory said regime troops and allied paramilitaries retook control of the area of Ketlet al-Jamae in Mallah Farms.
Al-Assad's forces have been trying for months to recapture Castello Road.
"The rebels are engaged in fierce battles in their attempt to take back Ketlet al-Jamae because it is an important area for them," observatory head Rami Abdel-Rahman said.
Activists inside Aleppo said that opposition rebels had launched a counter-attack to dislodge regime forces from the area.
The violence comes a day after the Syrian army announced that a three-day ceasefire went into effect and would continue until midnight on Friday (2100 GMT Friday).
The observatory said that the truce was meant for the entire war-torn country.
The ceasefire coincided with the start of the Eid al-Fitr holiday, which marks the end of Islam's holy month of Ramadan.
"This is a fake ceasefire," said Mahmoud al-Shami, an activist based in Aleppo. "This is a new manoeuvre by the regime to score military advances," al-Shami told dpa.
Al-Assad's forces have made territorial gains since September when allied Russia started an air campaign in Syria.
Meanwhile, US-backed Syrian forces made some important advances against Islamic State militants south of the city of Minbij in the north of the country, the Observatory said.
The US-backed Democratic Forces of Syria (DFS) - comprised largely of the Kurdish People's Protection Units (YPG), but also including Arab and Turkmen groups - are carrying out heavy battles in the neighborhood of al Hazwana and the so-called Al Shariya School in southern Minbij.
"The DFS has infiltrated deep into the city and managed to control the southwestern part of Minbij and is now only one kilometre from the city centre," the Obsevratory said.
Taking control of Minbij would cut off Islamic State from the Turkish border - just 20 kilometres from Minbij city - and supply routes, while also isolating the militant organizations de facto capital al-Raqqa.