David Cameron says he is determined to give Syria a brighter future as he met refugees of the conflict at a camp on the Syrian-Jordanian border.
Britain is to begin talks with armed Syrian rebels in a bid to unite the opposition to President Bashar al-Assad, the UK prime minister said.
The discussions are due to take place in Jordan and Turkey.
Visiting the refugee camp on Wednesday, Mr Cameron said the suffering of refugees was “horrendous”.
Up to 500 people a day have been arriving at the Za’atri camp, Mr Cameron was told, some having walked 15 days in order to try and find shelter.
Mr Cameron, who flew into the camp by helicopter, said UK humanitarian support for the victims of the 18-month conflict in Syria would be increased by £14m to more than £50m – making it the second largest donor after the United States.
David Cameron says he would like President Assad to see “full international justice”
It is thought that Mr Cameron is the first leader of a G20 country to visit Syrian refugees in Jordan to see the conditions they face.
“I wanted to hear for myself the stories of people who have been bombed and shot and blasted out of their homes in Syria by a deeply-illegitimate and unpleasant regime that is raining down death and destruction on its own people,” he said.
“It is truly horrendous to hear those stories and just redoubles my determination that now, with a newly-elected American president, we have got to do more to help this part of the world, to help Syria achieve transition.”
Foreign Secretary William Hague is said to have given his special envoy to the Syrian opposition, John Wilkes, the go-ahead to arrange the meetings with rebel groups opposed to the Assad government.
A spokesman said Britain would not be arming the Syrian rebels, or giving them access to military advisers.
Mr Cameron added: “There is an opportunity for Britain, for America, for Saudi Arabia, Jordan and like-minded allies to come together and try to help shape the opposition, outside Syria and inside Syria, and try to help them achieve their goal, which is our goal of a Syria without Assad.”
The conflict has been high on the agenda for Mr Cameron’s talks with the leaders of Saudi Arabia and the United Arab Emirates during his three-day trip to the Gulf.
He said on Tuesday that he was prepared to see President Assad allowed safe passage out of Syria if that would help ensure a peaceful power shift. But Mr Cameron insisted he would “favour him facing the full force of international law and justice for what he’s done.”
No 10 said the talks with Syrian rebels would help the UK “better understand the actual situation and the relationship between political and armed opposition groups”.
A spokesman said: “The government will make absolutely clear to these groups that they must respect human rights and humanitarian law standards. We will also call on them to work with aid agencies to facilitate vital humanitarian access.”
More than 30,000 people have been killed in the violence in Syria which began last year.
An estimated 2.5m people in the country need humanitarian assistance, and the number of refugees in the surrounding region is predicted to almost double to 710,000 by the end of December.
On the final day of his trip to the region, Mr Cameron will also hold talks with the King of Jordan.