UN inspectors tasked with investigating the alleged use of chemical weapons in Syria arrived Sunday in Damascus as fierce fighting raged in the coastal heartland of President Bashar al-Assad's clan.

The fighting in Latakia came as Assad vowed again that he was determined to "eradicate terrorism" which he blames for the deadly conflict that has plagued his country for the past 29 months.

The conflict has fuelled a massive exodus out of Syria, with nearly two million refugees seeking shelter in neighbouring countries and thousands more crossed into Iraq in recent days, the UN said.

A team of more than 10 inspectors arrived at the Four Seasons hotel in the Syrian capital to begin their hard-won mission which UN officials have said will last two weeks.

The mission had been repeatedly delayed over differences with Assad's regime concerning the scope of the probe into the alleged use of chemical arms in the Syrian war.

Both the government and the rebels fighting to overthrow Assad accuse each other of using chemical weapons.

The regime admitted in July last year for the first time that it has chemical weapons, threatening to use them to protect the country against Western military intervention but "never against the people."

At the time, US President Barack Obama warned the use of chemical weapons was a "red line" that would constitute a "game-changer."

On Thursday Damascus said it has "nothing to hide."

The UN team is led by Swedish arms expert Aake Sellstroem and is expected to investigate Khan al-Assal, near the northern city of Aleppo.

The government says rebels used chemical weapons on March 19, killing at least 26 people, including 16 Syrian soldiers, but the opposition says government forces carried out the attack.

Two other sites -- Ataybeh near Damascus and Homs in central Syria -- are also expected to be inspected for attacks that reportedly took place in March and December respectively.

The mission is tasked to assess if chemical weapons were used during the conflict that erupted in March 2011 -- but not to determine responsibility for any such attacks.

"Our goal remains a fully independent and impartial inquiry," the UN said on Wednesday after receiving a green light from the Syrian government for the mission.

"The government of Syria has formally accepted the modalities essential for cooperation to ensure the proper, safe and efficient conduct of the mission," UN Secretary General Ban Ki-moon's spokesman said at the time.

On the battlefield, fierce fighting raged in the coastal province of Latakia as the army pushed an advance to recapture villages seized by rebels in the hinterland of Assad's Alawite minority community.

Rebels positioned in remote enclaves of Latakia's mountains launched the "battle for the liberation of the Syrian coast" about two weeks ago.

Rami Abdel Rahman of the Syrian Observatory for Human Rights said the army sent "massive reinforcements" to Latakia to fight the rebels and "bombed rebel areas heavily."

State television said the army reclaimed rebel positions in the province, including Kharata, Janzuriyeh, Baluta, Baruda and Hambushiyeh.

But according to Abdel Rahman, "the army has only been able to secure the outskirts of some villages. The battles are ongoing and they are fierce."

"Scores of foreign (jihadist) fighters are being killed in the Latakia fighting," he told AFP, including a Libyan emir or local leader of the jihadist Islamic State of Iraq and the Levant.

The air force also struck several rebel positions in other parts of Syria on Sunday, a day after violence killed at least 124 people nationwide, the Observatory said.

Opposition National Coalition chief Ahmad Jarba, meanwhile, told a Saudi newspaper that rebel fighters control nearly half of Syria and the coming months would be "decisive."

On the humanitarian front, the UN said more than 15,000 Syrian refugees, mostly Kurds escaping deadly clashes between Kurdish fighters and jihadists, have crossed into Iraq since Thursday.

"UNHCR is witnessing a major exodus from Syria over the past few days unlike anything we have witnessed entering Iraq previously," said Claire Bourgeois, the Iraq representative of the UN refugee agency.

Source: AFP