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Syrian regime forces and Hizballah-led militias have  broken a three-year rebel siege of two Shia-majority towns in Aleppo province, leading to a new humanitarian crisis in northern Syria.

Local activists say the battle for the two towns are not yet lost, and fighting between rebel and regime forces have continued. 

Other reports suggest that Damascus has cut off a main supply route for the rebels and encircled opposition territories in Aleppo, home to hundreds thousands civilians.

The regime's capture of Nabul and Zahraa would also isolate rebel-held parts of Aleppo city from the Aleppo countryside and Azaz near the Turkish border, owing to the capture of strategic highway 214.

Ultimately this would mean fighters and civilians would be cut off from a crucial supply route to Turkey and countryside areas, placing up to 400,000 civilians under a de-facto siege.

A blockade on fuel and basic supplies - which the regime has carried out in dozens of civilian areas in Syria - would leave hundreds of thousands without access to electricity, heating and food, already in dire shortage.

The Syrian regime and its allies, backed by relentless Russian airstrikes, are leading a major offensive in Aleppo with a view to encircle rebel-held parts of the city.

One of the major goals of the offensive has been to break a rebel siege Nabul and al-Zahraa, where loyalist forces have been garrissoned for the past three years.

Hizballah-affiliated described the regime's encirclement as a major victory for the regime. 

Syrian opposition have accused Damascus of launching offensives in recent weeks to strengthen its hand in peace talks with the opposition.

Representatives from the opposition and regime are currently in Geneva, and talks expected to begin at the start of January have been postponed until later this month.

Relations between the two parties have been further strained by the offensive in Aleppo.

The official news agency SANA said army units together with Popuar Comittees - the name used by the regime for loyalist militias - are in the process of breaking the siege of Nubul and Zahraa imposed by "terrorist elements".

Mansour Hussein, a local activist, told °®Âþµº that regime forces have reached the two towns.

"Battles are still raging between the two sides in Maarasta al-Khan and Bayanon," he added.

"Several opposition fighters have been killed including a local Nusra Front commander, while thousands of families have started fleeing in the direction of the Turkish border." 

The military offensive around the city of Aleppo by Syrian government and allied forces has already uprooted hundreds of families in nearby towns.

Three humanitarian aid workers were also killed, the United Nations confirmed on Wednesday.

"The UN has received reports of displacement of hundreds of households in northeast towns of Bayanon, Hariyatan, Anadan, Hayyan and Ratyan of Syria following an unprecedented frequency of airstrikes in the past two days," a UN spokeswoman said in a statement to Reuters.

The towns of Nubul and Zahraa are strategically valuable areas given their close proximity to a highway which links the major Syrian city of Aleppo with Turkey.

The two towns, located in northern Aleppo, have been under siege since July 2012. Almost 45,000 people are estimated to have been trapped in these villages.

The Syrian regime currently imposes sieges on over 50 rebel-held areas, according to some estimates.