Care International warns that a large-scale regime offensive on Idlib would be catastrophic amid fears an assault could trigger a wave of 2.5 million refugees.
A Syrian regime offensive in the rebel-held enclave of Idlib would have a "catastrophic" humanitarian impact and cause levels of civilian suffering unprecedented in the seven-year war, aid agencies have warned.
The regime has said it aims to recapture the northwestern Idlib region, a refuge for civilians and rebels displaced from other areas of Syria as well as powerful militant forces.
"If there is a large-scale military offensive, what we might see in Idlib in terms of death, injury or displacement is something we haven't seen before. The war is far from over," said Joelle Bassoul, spokeswoman for Care International.
"People will be stranded with nowhere to go, with no aid -what other word can we use besides catastrophic," she told the Thomson Reuters Foundation by phone from Beirut on Tuesday.
Her comments came a day after the World Health Organization (WHO) warned of rising rates of acute malnutrition in Idlib and said a failure to vaccinate could lead to outbreaks of disease such as polio.
"The health situation in northwest Syria is already dire and looks set to deteriorate," said WHO Regional Emergencies Director Michel Thieren in a statement on Monday as the organisation appealed for more funds.
"If WHO does not receive additional funding, more than two million people caught in the cross-fire may have no access to essential health care services, including life-saving trauma care."
Bassoul said healthcare facilities in Idlib were already overstretched and would be unable to cope with a large military offensive.
Northwestern Syria, where Idlib is located, is the last big area still in the hands of fighters seeking to overthrow regime leader Bashar al Assad, after the regime recaptured the area around Damascus and the southwest earlier this year.
Syrian regime forces dropped leaflets over Idlib earlier this month, urging people to agree to a return of state rule and telling them the war was nearing its end.
Many live in overcrowded tents and shelters with little access to clean water, food and proper healthcare, according to aid agencies.
The war has already killed hundreds of thousands of people and driven 11 million from their homes, and the United Nations has expressed worries that an offensive in Idlib could force 2.5 million people towards the Turkish border.
"There are no homes or other forms of shelter left really, even tents," said Amany Qaddour, regional director of the humanitarian organisation Syria Relief and Development, in a statement.
"Many are prepared to stay (in Idlib) as they have no other options at this point."