Aid organisation Oxfam warned on Monday that the warmer weather will increase health-related risks for hundreds of thousands of Syrian refugees in Jordan and Lebanon, and appealed for urgent funds.
With temperatures set to soar up to 40 degrees Celsius (104 Fahrenheit), and as refugees continue to flee to Jordan and Lebanon, "the health risks must be urgently addressed," Oxfam said in a statement.
"Increased cases of public health-related diseases such as diarrhoea and skin infections have already been recorded in host communities and temporary settlements where an increasing number of refugees now live," it added.
Oxfam said it is "crucial to ensure funds are in place to provide refugees with shelter, clean water and adequate sanitation," adding that it is aiming to raise $53.4 million (41.6 million euros) over the next year.
So far Oxfam's appeal is only 23 percent funded, the statement said.
The UN High Commissioner for Refugees has said the number of refugees in Jordan alone could reach 1.2 million by the end of the year -- equivalent to one fifth of the kingdom's population.
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The kingdom says it currently hosts more than 500,000 Syrian refugees.
Lebanon currently houses 400,000 Syrians, according to the United Nations.
"Providing safe water, appropriate sanitation facilities and access to health care is costly. It's time for the key donors to wake up and face that reality," said Rick Bauer, regional humanitarian coordinator of Oxfam.
"The sad reality is that the vast majority of Syrian refugees are not going home soon. It is also crystal clear that host communities in Lebanon and Jordan need urgent help."
Bauer said Oxfam is "starting to really worry about the health of Syrian refugees".
"The aid effort must be properly funded and focused on providing refugees with affordable and decent places to stay, where they can live with dignity. That's priority number one for refugees and host communities alike," he said.
Last month, Oxfam urged the UN Security Council to help improve humanitarian access to war-torn Syria, saying that more funds are needed as the "catastrophe worsens".