Ministers on Wednesday night allocated an extra £87m to the UN World Food Programme to reach a 230,000 more Syrian children and their families over the next six months.
Justine Greening, the international development secretary, made the announcement as the UN security council unanimously agreed a new statement calling for unfettered humanitarian access to the war-torn country.
UK officials said the Syrian humanitarian crisis has reached catastrophic proportions, even though there has been diplomatic progress on chemical weapons and, in principle, humanitarian access.
The British government has so far set aside £500m for the crisis, including an extra £100m announced by the deputy prime minister, Nick Clegg, at the UN general assembly last week.
The Department for International Development expects the £87m will enable food supplies to reach more than 230,000 Syrian children, half of whom are refugees, each month for half a year. A further 200,000 stuck in camps on the borders of Syria will be offered food vouchers.
UK funding to the World Food Programme in Syria and the region now totals more than £150m, and the WFP estimates that it costs £11 to feed a Syrian family for a week. The UN agency is aiming to feed 3 million people in Syria every month and plans to further scale up its operations to reach 4 million during October.
Prices for everyday items and services have spiralled upwards in Syria owing to the civil war. A kilogramme of chicken in Damascus costs 10% of an average salary.
It is estimated there are 6.8 million people in need within Syria including 4.25 million forced to flee their homes to other parts of the country. Half of the refugees are under 18.
Earlier tSpeaking in Geneva this week, Muhannad Hadi, the WFP's emergency regional co-ordinator for Syria, said: "The agency is concerned with the ever-increasing number of displaced people and the fact that all sectors of the economy are collapsing.
"People who may have had income cannot purchase food due to high inflation, he noted. Syria had also experienced the worst harvest in years."
Ministers, thwarted by the Commons from backing military action against the Assad regime for its use of chemical weapons, have focused on improving the amount and quality of aid to Syrian refugees, as well as the speed with which it reaches civilians trapped inside the country.
Separately the security council agreed at meeting in New York to issue a resolution calling for the Syrian authorities to "lift bureaucratic restrictions that prevent lifesaving aid from reaching civilians – in particular by granting visas for NGOs and UN staff, expediting registration for international NGOs, and facilitating access for relief convoys across conflict lines and international borders".
It also urges all parties to the conflict to immediately demilitarise medical facilities and schools as well as agree on humanitarian pauses in the fighting to facilitate safe and unhindered access. A coalition of 16 humanitarian agencies called on all sides to enforce the resolution as quickly as possible.
Responding to the resolution, Greening said: "It is very welcome news that the UN security council has agreed on the need for unhindered access to all those needing humanitarian support in Syria. It is vital that aid can get through as quickly as possible to those who need it most."
Save the Children's chief executive, Justin Forsyth, said: "This as an important first step but we know from past conflicts how easy it is for agreements to be made on paper but no action to be delivered on the ground.
"The fight to save Syria's children is not yet won. The serious work begins now; following this agreement we need the international community to commit to a concrete delivery plan to provide millions inside Syria with food, medicine and shelter."
