text: They said the Syrian government had failed to respect the 1 June deadline for widespread aid distribution agreed by world and regional powers.
  Only a small amount of aid was delivered on Wednesday and a convoy to the town of Darayya near Damascus did not carry food.
  The UN Security Council will meet on Friday to discuss the air drops.
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  US State Department spokesman John Kirby said hundreds of thousands of Syrians needed "sustained and regular" access to aid.
  He said the UN food agency, the World Food Programme, had briefed the US on how the air drops could be carried out.
  Meanwhile British Foreign Secretary Philip Hammond described the limited access for aid allowed on the day of the deadline agreed by the International Syria Support group (ISSG) as "cynical".
  "While air drops are complex, costly and risky, they are now the last resort to relieve human suffering across many besieged areas," he said.
  The US and UK called on Russia and Iran, which back the Syrian government, to use their influence to ensure the air drops could proceed safely.
  Russia said the arrival of Wednesday's aid convoys was a positive step.
  The ISSG, co-chaired by the US and Russia, comprises 17 world and regional powers, as well the Arab League, the European Union and the United Nations.
  In April, the UN said at least 4,000 people were besieged in Darayya by Syrian government forces, which have surrounded the suburb for four years.
  The Darayya convoy - the first since November 2012 - contained vaccines, baby milk, medicine and nutritional goods.
  Separately, the rebel-held town of Muadhamiya, north-west of Darayya, received deliveries of food parcels and wheat flour on Wednesday, a month since aid convoys last visited.
  Darayya's electricity supply was cut off more than three years ago.
  UN emergency relief co-ordinator Stephen O'Brien said in April that the Syrian government had ignored "countless" requests for aid to be allowed in.
  An aid convoy was blocked from entering the town last month, despite all involved parties agreeing aid could be delivered.
  The latest delivery was made by teams from the UN, the International Committee of the Red Cross and the Syrian Red Crescent.
  Russia's defence ministry said on Wednesday it had agreed a pause with the Syrian authorities for 48 hours to allow for the distribution of humanitarian aid in Darayya.
  It borders a military airport used by Russian planes in support of the government of President Bashar al-Assad.
  It was one of the first towns to report demonstrations against the Assad regime, and has been under siege since late 2012. The UK-based Syrian Observatory for Human Rights reported clashes this week between rebel groups and government fighters on the outskirts of Darayya.
  The UN believes there are more than 4.6m people living in hard-to-reach areas in Syria, including nearly 600,000 in besieged areas.
target: The US, UK and France have urged the UN to begin air drops of humanitarian aid to besieged areas in Syria.