WASHINGTON (AFP) - Fresh from visiting a huge refugee camp, US Secretary of State John Kerry on Tuesday huddled with United Nations (UN) and aid organisation leaders to discuss ways to help millions caught in the crossfire in Syria.

With no end in sight to the conflict now in its 28th month, the grim toll keeps rising - some 100,000 people have been killed and 1.8 million Syrians are now refugees in neighboring countries.

Up to four million people are also now believed to have been displaced by the fighting inside Syria's borders, where local aid workers risk their lives daily to cross shifting frontlines to supply vital food and water.

Last week, Mr Kerry flew over the sprawling Zaatari refugee camp in northern Jordan, home to some 115,000 refugees and was briefed on the daily struggle for families - mostly women and children - many of whom have witnessed terrible trauma and brutality.

"We are having a very difficult time being able to access people, move people, protect people," Mr Kerry said at the start of the high-level talks in the State Department.

UN and non-governmental organisation (NGO) leaders say the Syrian conflict is the worst they have seen since the genocide in Rwanda in 1994.

Fears are growing that the conflict is spilling over the country's borders, destabilising already vulnerable neighbors such as Jordan, Lebanon and Iraq.

"We intend to have a very solid, in-depth discussion today about creative ways that we can meet our obligations to human beings who are in huge danger," Mr Kerry added.

He was meeting with the UN High Commissioner for Refugees Antonio Guterres as well as the heads of the World Food Programme, Unicef and the International Red Cross.

Mr Kerry thanked all the aid workers "for their courage" in helping to get vital supplies to Syrians both inside and outside of the country.

His visit to Zaatari had been "unbelievably moving", Mr Kerry said, praising how the refugees "somehow try to pull themselves together".

"They need the help of the world and it is my privilege to meet today with the people providing that help."

The United States is the largest single donor to UN-run relief programmes, having already pledged some US$815 million (S$1 billion) to help the Syrian refugees.

But unlike in natural disasters, such as earthquakes and tsunamis, charities and NGOs say appeals for donations from the public are largely falling on deaf ears.

With President Bashar al-Assad appearing to make some gains on the ground, and the Syrian opposition still struggling to overcome its disunity, aid organisations are digging in for a long, protracted war.

They now have to consider longer-term planning for housing and feeding millions of refugees, educating children and caring for daily medical needs, even as they deal with the daily crisis of some 6,000 people fleeing the fighting.

Last month, the United Nations launched its largest-ever appeal, calling for US$4.4 billion in donations to help the Syrian people.

"That's more than half the combined total of all of our other appeals which cover 16 countries from Afghanistan through to Somalia," UN under secretary for humanitarian affairs Valerie Amos said at the time.

She urged international efforts to find a political solution to end the fighting.

But moves to organise a peace conference between the regime and the opposition have so far faltered, with the opposition unable to agree on its representatives and disagreement between Russia and the United States on which countries - notably Iran - should sit at the table.

It had been initially hoped the conference could be held in Geneva in May, but the earliest estimates are now for some time in September.

UN-Arab League peace envoy Lakhdar Brahimi said on Monday he was pressing ahead with plans to hold peace talks.

But he acknowledged the difficulties.

"It is extremely difficult to bring people, who have been killing one another for two years, just by a magic wand to a conference like this. It will take time, but I hope it will happen," he told reporters.