Syria has agreed to allow UN investigators to visit three sites to investigate accusations of chemical weapons use during the country's two year civil war, the United Nations said.

The announcement on Wednesday came as President Bashar al-Assad forces launched an assault on Khan al-Asal, an Aleppo village that was recently captured by rebels and a site of an alleged chemical weapons attack.

"The Mission will travel to Syria as soon as possible to contemporaneously investigate three of the reported incidents, including Khan al-Asal," UN Secretary-General Ban Ki-moon's press office said in a statement.

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UN spokesman Martin Nesirky said the green light for the investigation followed last week's visit to Damascus by UN disarmament chief Angela Kane and the head of the chemical weapons investigation team, Ake Sellstrom, and "the understanding reached with the government of Syria".

The government and rebels blame each other for a purported March 19 chemical attack on Khan al-Asal that killed at least 30 people.

Nesirky did not give any details of the other two incidents to be investigated.

On Wednesday, regime troops attacked rebel positions outside Khan al-Assal, while fierce fighting erupted on the outskirts of the village, which the rebels seized on July 22, inflicting heavy losses on the army.

The army reportedly lost 150 soldiers over two days, 50 of them summarily executed after their capture, in an act condemned by the mainstream opposition leadership.

Regime advances

In the central city Homs, regime troops continued to bombard rebel-held neighbourhoods there.

Few days ago, Assad’s forces managed to take control of al-Khalidiyeh, a district in the city that had for more than a year been an opposition stronghold.

Fatih Hassoun, the Homs Front Commander in the Free Syrian Army, told Al Jazeera that the fighters retreated from the area as a result of the harsh conditions they lived in.

The Syrian army has been making advances in Homs city

"The decrease in the numbers of fighters and the many injuries they suffered, in addition to the blockade which was chocking old Homs for more than a year, led to the retreat of our fighters," Hassoun said.

Hisham Jaber a retired Lebanese Army General told Al Jazeera he believed the fall of the central city in the hand of regime troops was inevitable.

"Homs the city I think it will be taken. The battle will be over within a few day, maybe one week or two weeks," he said.

"We have to remember that we have 8,000 civilians inside Old Homs. [Within the armed] opposition, there is lack of organisation, lack of cooperation and lack of centralisation."

The Syrian army has been pushing steadily north in recent weeks. With the help of Hezbollah fighters from Lebanon, it took full control of the city of Qusayr in June.

Source: Al Jazeera and agencies