Medecins Sans Frontieres says about 60,000 Syrian refugees stuck in the desert on the Jordanian border have received no aid for more than a week.
The Jordanian authorities declared the border a closed military zone after a bomb attack near the informal camp at Rukban killed seven security personnel.
Since then, no food or medical assistance, and only extremely limited water, have reached the refugees.
Medecins Sans Frontiereswarned that the situation at Rukban was getting worse by the hour.
Conditions were already extremely harsh, with 214 of the 1,300 children under five years old screened by MSF teams malnourished and 24.7% of the children they saw suffering from diarrhoea.
Before the attack, Jordan was allowing in only about 50 to 100 refugees from Rukban and another camp at Hadalat each day, citing security concerns.
Jordanian government spokesman Mohammed Momanisaid refugees stranded at Rukban are dependent on aid, but that the camp had become an "enclave" for the militant group (IS) and that "national security must take precedence".
IS said one of its militants carried out the attack on 21 June, which saw a suicide bomber drive an explosives-laden lorry at the military post.
Many of the refugees at the camp are also believed to have fled areas of Syria controlled or previously controlled by IS. The group has come under pressure over the past year from Russian-backed forces loyal to President Bashar al-Assad and a Kurdish-dominated alliance supported by the US.