The UN General Assembly is poised to approve an Arab-backed resolution calling for a political transition in Syria and strongly condemning president Bashar Assad's regime for its escalating use of heavy weapons, UN diplomats said.

The 193-member world body is due to vote on the resolution which also condemns Syria's "gross violations" of human rights, assembly spokesman Nikola Jovanovic said.

The Arab group decided to seek approval of a wide-ranging resolution on Syria in the General Assembly, where there are no vetoes, to reflect international dismay at the increasing death toll, now more than 70,000, and the failure to end the more than two-year-old conflict.

The resolution will not have as much support as the assembly's previous resolution last August denouncing Syria's crackdown on dissent, diplomatic sources said.

Unlike Security Council resolutions, which are legally binding, General Assembly resolutions cannot be enforced. But approval of an assembly resolution would counter the paralysis of the deeply divided Security Council, where Syria's allies, Russia and China, have vetoed three Western-backed resolutions aimed at pressuring Assad to end the violence.

Russia's UN ambassador Vitaly Churkin has sent letters urging all UN members to vote "no" on the new resolution. He called it "one-sided and biased", as well as "counter-productive" given the understanding reached by Russian foreign minister Sergey Lavrov and US secretary of state John Kerry in Moscow earlier this month to convene a follow-up international meeting on a political transition in Syria.

Argentina's UN ambassador Maria Cristina Perceval asked Qatar, the lead sponsor, to water down language welcoming the establishment of the Syrian National Coalition, the main opposition group, and to eliminate a reference to "the wide international acknowledgement" that it is the legitimate representative of the Syrian people.

She said in a letter, written on behalf of a group of Latin American countries, that they also objected to language welcoming Arab League proposals on a political resolution of the situation in Syria because Latin American countries did not participate in their adoption. Ms Perceval also demanded that language be inserted "explicitly calling for no further militarisation of the conflict".

The Qataris refused to make any of these requested changes, putting support from this group of Latin American countries in doubt.

The last Arab-sponsored General Assembly resolution in August was approved by an overwhelming vote of 133-12 with 31 abstentions. It strongly condemned Syria's use of heavy weapons, deplored the Security Council's failure to act, said Syria must take the first steps to stop the violence, and stressed that rapid progress on a political transition represented the "best opportunity" to resolve the crisis peacefully.

Diplomats predicted that the new resolution would get a reduced majority of between 100 and 110 "yes" votes because of concerns about the divided opposition.

Using similar language to the August resolution, the proposed new resolution stresses that "rapid progress on a political transition represents the best opportunity" to resolve the Syrian conflict peacefully.

It would promote the road map for a Syrian transition adopted at a meeting on June 30 last year in Geneva by key nations including the five veto-wielding council powers - the US, UK, Russia, China and France. The road map starts with the establishment of a transitional governing body vested with full executive powers and ends with elections - but there has been no agreement on how to implement it, which would require Assad to relinquish power at some point.

The draft resolution would reiterate the General Assembly's call "for an inclusive Syrian-led political transition to a democratic, pluralistic political system, in which citizens are equal regardless of their affiliations or ethnicities or beliefs". It says this should be done by starting "a serious political dialogue between credible, empowered and mutually acceptable interlocutors representing the Syrian authorities and the Syrian opposition".

The draft strongly condemns the continued escalation in the Syrian regime's use of heavy weapons, including indiscriminate shelling from tanks and aircraft, as well as the use of ballistic missiles, cluster munitions and other weapons against populated areas.

It expresses "grave concern at the threat by the Syrian authorities to use chemical or biological weapons, as well as at allegations of reported use of such weapons". It demands that Syria "strictly observes" international laws prohibiting the use of chemical and biological weapons and refrains from transferring such weapons "to non-state actors".

Syria has said if it had such weapons, it would never use them against its own people.

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