The UN urged governments on Wednesday to unlock the funds it desperately needs to assist one million refugees who have fled Syria's war, which a charity said increasingly features child soldiers.
Russia, meanwhile, warned that arming Syria's rebels would breach international law, after Western powers dropped hints about giving military aid.
UN High Commissioner for Refugees Antonio Guterres warned of the "enormous threat" to the region from the two-year conflict that has already killed at least 70,000 people.
"I appeal to parliaments, governments to approve extraordinary funds to support the Syrian victims and the countries that receive them," Guterres said in Jordan, which hosts 450,000 refugees, almost half of the total.
"If that does not happen, with the normal UN aid budgets we will not be able to deliver," he said, urging world powers to press for a solution or face the "catastrophic scenario" of the conflict spilling over borders.
His plea was echoed by Joel Charny, vice president of the US-based alliance of relief organisations, InterAction, who urged donors to make good on promises made at a conference in Kuwait in January.
So far only around 20 percent of the $1.5 billion pledged has been received, according to figures from the UN Office for the Coordination of Humanitarian Affairs.
The appeals came a day after Syria said its army was ready to fight "for years" and ahead of the second anniversary of the March 15, 2011 nationwide protests that triggered the conflict.
The demonstrations broke out after the arrest and torture of children accused of painting anti-regime graffiti in the southern city of Daraa.
Two years on, a British charity reported that children are being increasingly recruited on the front line, with both sides using boys as soldiers and even human shields.
"There is a growing pattern of armed groups on both sides of the conflict recruiting children under 18 as porters, guards, informers or fighters," said Save the Children.
In a report on Tuesday, the UN children's agency said children have been recruited in a conflict it says threatens an entire generation.
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UNICEF said nearly two million children in Syria under 18 were in dire need of aid, about 800,000 under 14 were internally displaced and that more than 500,000 children have fled the violence as refugees.
In London, meanwhile, Russian Foreign Minister Sergei Lavrov met his British counterpart William Hague a day after Prime Minister David Cameron said Britain would consider ignoring an EU arms ban and supply weapons to the rebels.
"Arming the opposition is in breach of international law," Lavrov said at a news conference with Hague, British Defence Minister Philip Hammond and Russian Defence Minister Sergei Shoigu.
Western powers have stepped up non-military support for the rebels, even as Russia has continued to arm its ally Assad.
Britain is currently giving "non-lethal" support to the rebels but Hague and Hammond refused to rule out arming them.
"What you can be assured of is that any action we take will be legal, will be clearly with a strong basis in international law," Hammond insisted.
An opposition National Coalition spokesman, meanwhile, told AFP the opposition is to meet next Monday and Tuesday in Istanbul for delayed talks on forming an interim government and electing a prime minister for rebel-controlled areas.
On the ground, a regime air strike targeted the strife-torn district of Baba Amr in Homs after rebels infiltrated the area, and clashes raged near the tense border with Lebanon, said the Syrian Observatory for Human Rights.
And 12 people were killed, including several children, by mortar fire in Damascus, said the Observatory, which reported an overall death toll of at least 130, including 52 civilians, in violence on Wednesday.
At the United Nations, diplomats said the world body has halted peacekeeping patrols in the Golan Heights amid fears that fallout from the Syrian war could lead more countries to withdraw from its force.
The four-day abduction last week of 21 Philippine members of the United Nations Disengagement Observer Force, which has monitored a ceasefire zone between Syria and Israel since 1974, has heightened security fears.
Canada pulled its troops out in March 2006, while Japan and Croatia withdrew their contingents in recent months, leaving only the Philippines, Austria and India in UNDOF.