Amnesty UK has warned of the risk of mass starvation in Syria and has urged international community to give more help.
Syrians risk also to face severe shortages of medical care and adequate shelters.
Those who have fled to neighbouring countries also face tough conditions in poorly resourced refugee camps.
Philip Luther, director of Amnesty International's Middle East and North Africa Programme, said: "The world's response to the Syria crisis has been woefully inadequate. At the end of 2013 the UN humanitarian appeal - the largest in the organisation's history - was just 70%-funded.
"This meant that vital aid was cut off to some of the most vulnerable victims of Syria's brutal conflict who were left to face the bitter winter months with minimal resources.
"The world cannot repeat the mistakes of last year.
"Many of those displaced by the conflict either inside or outside the country are simply not receiving the help that they need. The international community must step up its efforts on all fronts to prove it has not forgotten about them," Luther continued.
Although some countries have made generous contributions, others such as Saudi Arabia and Russia have failed to fulfil their promises of aid, Amnesty said.
International efforts to resettle refugees were also slammed as "pitiful". European Union member states have pledged to resettle just 0.5% of the 2.3 million people who have fled the country.
A conference hosted by Amnesty is aimed at kickstarting the campaign to help with the launch of a US$6.5bn (£3.9bn) fundraising appeal.
According to the Syrian Observatory for Human Rights, the death toll in the conflict, which began almost three years ago, was over 130,000 and millions of civilians have fled their homes.
The UN announced that it would stop updating the death toll in Syria as it can no longer verify the accuracy of the sources.
A peace conference is due to start on 24 January at the UN headquarters in Geneva, with meetings between Assad's representatives and opposition groups.
Iran has not been invited to take part after the US and Russia failed to reach an agreement on its role.