International Development Secretary Justine Greening said the additional UK assistance announced today could provide food for nearly 23,000 people, clean water for 130,000 people and shelter for more than 44,000 within Syria.
It would also provide food for 21,000 Syrian refugees, clean water for more than 14,000 and shelter for 15,000 people in neighbouring countries.
It could also fund medical consultations for more than 35,000 people in Syria and 6,000 across its borders.
Ms Greening said:
The crisis in Syria has left millions of people needing urgent food supplies and medical attention, but the Assad regime is creating bureaucratic barriers that stop help getting where it is needed.
Inside Syria, medical deliveries have been blocked and aid workers attacked and killed.
G20 leaders must now speak with a single voice and send a clear message to all sides to this conflict.
Unless access improves, we risk a lost generation of Syrian children - the very generation who will be expected to build the peaceful, democratic Syria of tomorrow.
The G20 summit in St Petersburg exposed stark divisions between host Vladimir Putin and US President Obama over Syria.
War emptied the suburb of Yarmouk in southern Damascus of its people. Assad's army holds it, but their prize is a ghost town.
Even in the President's heartland of Baltimore, 95% of voters are against his policy on military intervention in Syria.