Saudi Arabia and Iran said on Sunday that an escalating dispute between the two countries would not affect international efforts to end the war in Syria, even as a large Syrian rebel group cast doubt on the United Nations-led peace process.

The U.N. envoy for Syria, Staffan de Mistura, said in a statement after meeting Iranian Foreign Minister Javad Zarif in Tehran that Iran had assured him that the row would not upset talks set for later this month.

De Mistura is shuttling around the region to shore up support for the negotiations, which are due to start in Geneva on Jan. 25. They are part of a plan endorsed by the Security Council last month to end the five-year-war that has killed 250,000 people and created millions of refugees.

Saudi Foreign Minister Adel al-Jubeir, attending an Arab League meeting on Sunday to discuss the spat between the two Gulf rivals, also said he did not expect the diplomatic row to affect peace efforts.

Tensions between the Sunni Muslim kingdom of Saudi Arabia and Shi'ite Muslim Iran have escalated since Saudi authorities executed Shi'ite cleric Nimr al-Nimr on Jan. 2, triggering outrage among Shi'ites across the Middle East. Iran backs the government of Syrian President Bashar al-Assad, while Saudi Arabia has provided support to Sunni rebels.

Syrian opposition officials have expressed misgivings about the peace talks, citing the need to see goodwill measures from the government side including a ceasefire, a detainee release and the end of blockades on besieged areas before starting negotiations.

Islam Army (Jaysh al-Islam), part of a newly formed council to oversee the negotiations on the opposition side, said in a statement that it was unacceptable to talk about a political solution to the war while people died of hunger and bombardment.

The group said the "best way to force the regime to accept the (political) solution and stick by it" was to allow states that back the opposition to supply rebels with anti-aircraft missiles.

The statement, sent by the Islam Army's spokesman overnight, said it would guarantee the missiles would not reach groups that would use them "illegally".

Foreign governments including the United States and Saudi Arabia have provided rebels with military support, but have resisted demands for missiles for fear they would end up with hardline jihadist groups such as Islamic State.

The Syrian government says Islam Army is a terrorist group, like all the groups that are fighting to topple Assad, who has received crucial support from Russia and Iran. Both states have sent forces to help him fight the insurgency.

The Syrian government told de Mistura on Saturday it was ready to take part in Geneva talks but stressed the need to see the names of the Syrian opposition figures who will take part.

Pointing to another potential complication, Foreign Minister Walid al-Moualem also stressed the need for the government to obtain a list of groups that would be classified as terrorists as part of the peace process.

Islam Army said the success of the political process "depended on the seriousness of the international community in putting pressure on the criminal regime to halt the killing".