David Cameron has announced an additional £52 million in UK aid for victims of the civil war in Syria.
The news came as the G20 summit in St Petersburg saw stark division over the international response to allegations of chemical weapons use by the regime of President Bashar Assad.
The prime minister had previously vowed the UK will "lead the world" in getting humanitarian aid to refugees in Syria.
Much of the new money, which will bring the total UK funding for assistance in Syria and neighbouring states to £400 million, will go towards medical training and equipment to help civilians targeted by chemical attacks.
Mr Cameron also called for international action to secure humanitarian access to war zones.
After defeat in the House of Commons prevented Mr Cameron from joining US President Barack Obama in preparing for military action against Assad, the Prime Minister has sought to avoid being marginalised at St Petersburg by putting himself at the head of a drive to persuade G20 leaders to strengthen their support for efforts to relieve the suffering of millions of Syrians affected by the two-year civil war.
The PM was yesterday forced to deny he was being sidelined, after reports that a senior aide to summit host Vladimir Putin - a long-time ally of Assad who has warned that military action without UN backing would amount to an act of aggression - had dismissed Britain as "just a small island: no-one pays any attention to them".
Downing Street demanded "clarification" of the remarks, which were denied by the Russian President's chief spokesman Dmitry Peskov, who said they did not reflect Moscow's true view of the UK and the "positive dynamics" of relations between the two countries.
A Number 10 source said that the reported remark "highlights how a small island with great people can achieve a big footprint in the world".
It is not known whether the alleged snub was discussed in a 35-minute bilateral meeting between Mr Cameron and Mr Putin which took place at 2am St Petersburg time, after the Russian President had treated his guests to a lavish display of music and fireworks at the Peterhof Palace.
Mr Cameron announced Britain's new aid contribution at a meeting in which he urged fellow leaders to dig deep to fund a $3bn (£1.9 billion) shortfall in the United Nations appeal for Syria.
Acknowledging the deep disagreements voiced when Syria was discussed at last night's official dinner, Mr Cameron said that action on aid would show that the G20 had "a very large message" of support for the people of the war-torn Middle Eastern state.
"This is a moral imperative," Mr Cameron told the meeting, attended by ministers from Canada, Japan, Australia, Saudi Arabia and Turkey and a senior official in the Obama administration, as well as UN Secretary General Ban Ki-moon and the EU's Jose Manuel Barroso and Herman van Rompuy.
"This is the big refugee crisis of our time.
"It is also a political imperative. It will help us build international support for action by showing that our response is not just military.
At a summit where people have focused on potential divisions over Syria, I wanted to bring you together to identify key priorities about the action needed to send a strong message about our commitment to the Syrian people and the urgent priority to do more."
Reminding fellow leaders that the UN relief appeal is so far only 43% funded, Mr Cameron said: "The UK is the second largest bilateral donor and today we can announce an extra 81 million dollars... We would welcome an announcement by others to highlight our resolve to do more."