The Syrian army is using bombs to cut the only road into rebel-held Aleppo

Syrian government forces effectively cut the only way into rebel-held areas of Aleppo on Thursday as they advanced to within firing range of the road, rebels said, putting some 250,000 people living there under siege.

The advance near the Castello Road brought the Syrian government closer to its long-standing objective of encircling rebel-held areas of Aleppo. The rebels however said a fightback was underway to take back lost positions and re-secure the road.

The advance came during a 72-hour ceasefire announced by the Syrian army on Wednesday.

Aleppo, Syria's largest city before the civil war with a population of more than two million people, has been divided for years into rebel and government sectors. The Aleppo area has been a major theater in the conflict.

Heavy aerial and artillery bombardment had at times made the Castello Road impassable. But Thursday's advance brings government forces the closest so far to the road, making it even easier to hit and effectively cutting off the opposition-held sector of the city near the Turkish border.

"Currently nobody can get in or out of Aleppo," Zakaria Malahifji of the Aleppo-based rebel group Fastaqim told Reuters.

The Syrian Observatory for Human Rights said the advance by pro-government forces in the al-Malah Farms area had brought them to within less than a mile of the road.

The army said what it described as terrorist groups had tried to attack army positions in the area, and that it thwarted this assault and taken over the southern al-Malah farms, coming to within firing distance of the Castello road.

A second rebel official with fighters in the area said: "All the factions sent reinforcements and are trying to take back the positions taken by the regime, but the situation is very bad. There was heavy regime air cover in the night."

The Observatory estimates that between 250,000 and 300,000 people live in opposition-held parts of Aleppo.

Syria's civil war, now in its sixth year, has killed at least 250,000 people, displaced more than 6.6 million inside the country and forced another 4.8 million to flee, many seeking refuge in Europe.

US Secretary of State John Kerry on Wednesday had welcomed the Syrian army's announcement of the truce, adding that discussions were under way to try to extend it.

President Bashar al-Assad's military hand has been strengthened by Russian air strikes that have been underway since September.

Last month, Assad's allies in the Iranian-backed Lebanese group Hezbollah said they would send more fighters to the Aleppo area. The group's leader said the defense of Aleppo was tantamount to the defense of Damascus.

Earlier this year, pro-government forces including Shiite militias severed a separate important route north of Aleppo into opposition-held parts of the city.

Assad's allies say they are battling the al Qaeda-linked Nusra Front in Aleppo. Groups fighting under the banner of the Free Syrian Army (FSA) say however that they control the rebel-held part of the city.

Some of the FSA groups have received military support from Assad's foreign enemies, including Turkey, and the United States.

The Nusra Front has meanwhile played a significant role in fighting against pro-government forces southwest of Aleppo.

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