Syrian President Bashar al-Assad has appealed to the leaders of a five-nation economic forum meeting in South Africa to help end his country's two-year conflict.

The embattled and increasingly isolated president said on Wednesday that his country is being subjected to "acts of terrorism backed by Arab, regional and Western nations", a reference to the Western-backed opposition fighting his regime.

Assad's appeal came in a letter sent to a forum of BRICS nations; Brazil, Russia, India, China and South Africa, whose leaders have gathered for a summit in Johannesburg.

The president's letter was published by Syria's state media on Wednesday.

It came a day after the Arab League allowed opposition leader Mouaz al-Khatib to fill Syria's vacant seat at the organisation's annual summit in Doha, the Qatari capital.

Syrian membership to the 22-member league was suspended in November 2011 in response to the government’s bloody crackdown on the opposition.

'Tainted oil and money'

Syrian government on Wednesday vetted its wrath at Qatar and the Arab League for handing its seat to the "deformed" Syrian National Coalition.

"The emir of Qatar, the biggest bank for supporting terrorism in the region, began his presidency of the Arab League by hijacking it with tainted oil and money," said state news agency SANA.

It said the League had compromised its values for the sake of Gulf Arab and Western interests.

Iran has also criticised the Arab League decision.

"Assigning Syria's seat to the Arab League to those who don't have the backing of the people establishes a pattern of dangerous behaviour for the Arab world that can set a new precedent for other members of the Arab League in the future," said deputy foreign minister, Hossein Amir Abdullahian, Iran's student news agency reported on Tuesday.

The summit endorsed the provision of military aid to Syrian rebels.

A communique affirmed member states had a right to offer assistance "including military, to support the steadfastness of the Syrian people and the Free Army".

However, "efforts aimed at reaching a political solution to the Syrian crisis are a priority," it added.

Khatib, the head of the Coalition, asked US Secretary of State John Kerry for American forces to help defend rebel-controlled northern parts of Syria with Patriot surface-to-air missiles based in Turkey.

NATO swiftly rebuffed the idea.

'Do what you want'

Khatib said NATO’s response sent a message to Assad's government to "do what you want".

In an interview with Reuters news agency, Khatib said that he would not rescind his resignation as leader of the Coalition - which he announced on social media last week - but would continue to perform leadership duties for the time being.

In a statement, Khatib said he was stepping down out of frustration from the international community.

"For the past two years, we have been slaughtered by an unprecedentedly vicious regime, while the world has looked on," Khatib said in the statement.

"All the destruction of Syria's infrastructure, the detention of tens of thousands of people, the forced flight of hundreds of thousands and other forms of suffering have been insufficient for the international community to take a decision to allow the people to defend themselves," he added.

The Coalition rejected his resignation.