"The body interested with maintaining international peace and security cannot be 'missing in action'," Mr Ban said.
"The council must at last find the unity to act. It must use its authority for peace," he went on.
"We've always said we want the UN Security Council to live up to its responsibilities on Syria," Mr Cameron said in another message.
"Today they have an opportunity to do that," he said. The draft resolution would condemn the "chemical weapons attack by Assad", he added.
In a briefing to journalists, joint UN-Arab League envoy to Syria Lakhdar Brahimi said: "It does seem clear that some kind of substance was used... that killed a lot of people" on 21 August. But he emphasised that any military action needed Security Council authorisation. 'Further destabilisation'Russia and China have previously vetoed resolutions critical of Syria and may block any text deemed to approve military action. Russian Foreign Minister Sergei Lavrov warned that "attempts at a military solution will lead only to the further destabilisation" in Syria and the region. Mr Lavrov emphasised the need for a political solution in a phone call to Mr Brahimi, the foreign ministry in Moscow said.
Russia, China and Iran have previously warned against launching an attack on the war-ravaged country, where more than 100,000 people are thought to have died in two years of fighting. Iran's Supreme Leader Ayatollah Ali Khamenei warned on Wednesday that US intervention would be a "disaster" for the region. "The region is like a gunpowder store and the future cannot be predicted," Mr Khamenei said, according to Iran's Isna news agency. Stocks have fallen on global markets and oil prices have shot up amid growing concern about an impending attack
Continue reading the main storyModels for possible intervention
- Iraq 1991: US-led global military coalition, anchored in international law; explicit mandate from UN Security Council to evict Iraqi forces from Kuwait
- Balkans 1990s: US arms supplied to anti-Serb resistance in Croatia and Bosnia in defiance of UN-mandated embargo; later US-led air campaign against Serb paramilitaries. In 1999, US jets provided bulk of 38,000 Nato sorties against Serbia to prevent massacres in Kosovo - legally controversial with UN Security Council resolutions linked to "enforcement measures"
- Somalia 1992-93: UN Security Council authorised creation of international force with aim of facilitating humanitarian supplies as Somali state failed. Gradual US military involvement without clear objective culminated in Black Hawk Down disaster in 1993. US troops pulled out
- Libya 2011: France and UK sought UN Security Council authorisation for humanitarian operation in Benghazi in 2011. Russia and China abstained but did not veto resolution. Air offensive continued until fall of Gaddafi
- Syria crisis: Western military options
- Models for possible intervention
- Press apprehension as Syria tension builds
- Syria crisis: Where key countries stand