David Miliband has urged the world to "put the humanity back" into the pursuit of peace in Syria as the conflict enters its seventh year.
The former foreign secretary, who heads up the International Rescue Committee (IRC), said too many civilians had lost their lives through the "extraordinary abuse of international law" during the bitter struggle.
He told the Press Association: "The message for the sixth anniversary is that 25 million refugees (worldwide) are 25 million people, and the great danger is that the statistics dehumanise the refugee population.
"These are doctors and dentists and business people and housewives and househusbands and people who are really trying to keep their lives together in the way that you or I do, but trying to do so in appalling circumstances.
"And I think that the message on the sixth anniversary of the Syria crisis is that 500,000 lives have been lost. That's not a statistic, that's 500,000 people whose families have been grotesquely affected by this.
"The sixth anniversary is a day to put the humanity back into the conduct of the war and into the pursuit of peace."
On March 15 2011 protests erupted in Damascus's Old City and the southern city of Daraa over security forces' detention of a group of boys accused of painting anti-government graffiti on their school walls - a day widely regarded as the start of the uprising.
Since then, more than half of Syria's pre-war population of 22 million citizens has been uprooted.
The majority of the 4.9 million Syrian refugees registered by the UN are overwhelmingly staying in neighbouring countries including Turkey, Egypt, Iraq, Jordan and Lebanon.
Charities including Oxfam have criticised the international community for its "increasingly restrictive" policies against Syrian refugees who are "seeing doors slammed in their faces".
US President Donald Trump recently issued an updated travel order banning travellers from six predominantly Muslim countries, including Syria, while the UK Government said the Dubs scheme offering legal routes to lone child refugees will end after 350 children have been brought to Britain.
Mr Miliband said that the president's ban "threatens to be a historic assault on a very successful American refugee resettlement programme".
Urging caution as the conflict moves into its seventh year, he said: "This is an extremely complicated region, where external engagement can be very influential and where all sorts of actors are now on the stage.
"It's very important that western countries take careful note of the different players and are very, very careful indeed of the kind of actions that can promote stability or undermine it, and that's something that needs to be central over the next few months."
International Development Secretary Priti Patel said that the UK had committed £2.3 billion since 2012 to humanitarian assistance for people in Syria and refugee camps in neighbouring countries like Lebanon, Jordan and Turkey.
The aid cash has funded more than 21 million monthly food rations, 6.5 million relief packages, 6.3 million vaccinations and almost 5 million medical consultations, said the Department for International Development.
Ms Patel called on the international community to "dig deep" to support Syria in a donors' conference which the UK is hosting in Brussels next month.
"Over the last six years, the UK has led the international response to the crisis in Syria, which has seen hundreds of thousands of lives lost, millions more thrown into chaos and a country systematically laid to waste," said Ms Patel.
"In a conflict where six years of barbarity have seen the targeting of civilians used as a weapon of war, we have ensured food, shelter and medicine is there for those caught up in the violence.
"Donors and host countries must keep up the pace to create more jobs and opportunities for the millions of people who continue to be affected by this crisis right across the region."