About 270,000 have fled, the UN said, with tens of thousands currently stuck by the border with Jordan after the Russian-backed government offensive began to recapture rebel-held southwestern Deraa province.
According to the UN, nearly half of the displaced are children.
“This is the largest population displacement in southern Syria since the onset of the crisis,” Anders Pedersen, UN resident and humanitarian coordinator in Jordan, wrote in a statement on Wednesday.
Jordan’s Foreign Minister Ayman Safadi said on Wednesday that dozens of trucks were waiting for Syrian permission to supply humanitarian aid from Jordan to Syria.
Safadi was speaking at a news conference following talks with his Russian counterpart Sergey Lavrov in Moscow.
After artillery strikes hit Jordanian territory from Syria, he said Jordan’s armed forces were ready to defend country’s interests.
Amman has kept its borders shut for the new wave of refugees from Deraa, as the country says it is unable to host additional Syrians.
However, the Jordanian government puts the number at 1.3 million, as the UNHCR only counts registered refugees, whereas Amman includes Syrians who have not received UN asylum.
The UN has dispatched humanitarian assistance over the past two months and has been helping to provide essential supplies such as food, water, soap and medical equipment with extra supplies prepared in case of further escalation in the conflict.
Children are at particular risk of dehydration and diarrhoea, Pederson noted.
“The United Nations reiterates its readiness to assist as much as needed but, at the same time, it also reminds all responsible parties that our effectiveness depends on facilitation of the provision of humanitarian aid and protection to people in need, in line with the parties’ obligations under international humanitarian law,” Pederson wrote.
“The United Nations reminds all states and parties involved in the conflict that a delay in responding to the worsening humanitarian crisis could only lead to catastrophic results.”
The UN’s concern comes as Safadi and Lavrov met on Wednesday to discuss the situation in Deraa, located near the Jordanian border.
Safadi told Lavrov that political dialogue and a ceasefire were priorities for southern Syria where he said a humanitarian catastrophe risked unfolding.
Deraa is located in one of the four “de-escalation zones“, as part of a Russian-led ceasefire deal for territory held by rebels. Russia, Turkey and Iran signed the deal in 2017 as guarantors.
Al Jazeera’s Rory Challands reporting from Moscow said that while Safadi arrived in Moscow hoping to persuade Lavrov in implementing some sort of a ceasefire, “he does not seem to have won that from the Russians.”
Lavrov and Safadi decided in the meeting to fulfil the de-escalation zone agreements, but Lavrov also added that the fight against “terrorists” in southern Syria remains a priority since they control about 40 percent, Challands said.
“That does not seem to me at the moment as if the Russians are willing to back off and there’s a good reason for that. This is a tactic that has worked elsewhere in Syria,” Challands said.
“We saw it in eastern Aleppo and in Douma – successful in both those places in winning territory back for the Syrian government. There was a huge military offensive in each of these places, massive Russian aerial bombardment that’s helped to create a humanitarian crisis on the ground, which in turn put more pressure on rebel groups in the area.
“Russia then told the rebel groups that they had to lay down weapons and surrender or vacate these areas for other areas of Syria. That is something that we are seeing played out again in Deraa; it’s worked for them before and there’s no reason to suspect that it won’t be able to work for them here as well.”