The Swiss-run International Committee of the Red Cross has launched an appeal for SFr100 million ($81 million) to help the victims of the ongoing conflict in Lebanon.
The ICRC said on Friday that life for civilians trapped by intense fighting in the south of the country was "rapidly becoming unbearable and the suffering excessive".
The Geneva-based organisation said access to water and basic services was extremely limited, and a key priority was reaching isolated populations.
Highlighting the critical security needs of civilians, ICRC director of operations Pierre Krähenbühl warned that the number of casualties was on the rise.
He said international humanitarian law set out clear rules and much more had to be done by Israeli forces to protect and spare civilians during military operations.
"In the south of Lebanon in particular, the number one issue today for us is the security of civilians and saving civilian lives, and securing access and safety for the medical and humanitarian missions," said Krähenbühl.
The ICRC noted that some aid convoys trying to reach villages had been forced to turn back due to intense fighting. While this was "deeply disappointing", Krähenbühl said there was no evidence of a systematic pattern of blocking access.
The organisation said it was also concerned about the plight of civilians in northern Israel, where a number of people have been killed or injured by rocket attacks. It says it is assisting medical services there.
To deal with the growing humanitarian crisis, the ICRC together with the Lebanese Red Cross is appealing for SFr100 million.
The funds will be used initially to expand health programmes and support medical facilities serving 650,000 people, as well as to support the treatment of up to 5,000 war-wounded.
The ICRC will increase deliveries of food and relief goods for at least 200,000 residents and those displaced by the fighting. The organisation is also seeking to improve access to clean water for more than 1.2 million people.
Balthasar Staehelin, the ICRC's delegate-general for the Middle East and North Africa, said aid workers were facing "a race against time" to reach villages in southern Lebanon cut off by the hostilities.
"There may well be a risk that in some of the villages, if we can't access them rapidly enough, people may die," he said. Staehelin recalled that Lebanese Red Cross ambulances had been damaged by Israeli fire and a number of first aid workers injured.
He said it was a "high concern" that the medical mission was respected, though he did not believe vehicles were being deliberately targeted in combat zones.
Earlier this week the Swiss government announced it was giving SFr5 million for the ICRC's emergency activities in Lebanon. The decision came at a special cabinet session where ministers controversially agreed to adopt a low profile on the conflict.
Foreign Minister Micheline Calmy-Rey, who had previously taken a strong line, said Switzerland – the depositary state of the Geneva Conventions – would now limit itself to humanitarian affairs.
The ICRC on Friday declined to get embroiled in the affair, which has divided opinion in Switzerland on whether the country should be taking a firm position on Lebanon.
swissinfo, Adam Beaumont in Geneva
The ICRC says it has expanded its staff in Lebanon to 60 personnel in Beirut, Tyre and Marjayoun.
Since hostilities broke out, the Lebanese Red Cross has evacuated and treated 571 casualties, transported 2,298 medical cases and collected 91 bodies.
The foreign ministry said on Friday that all Swiss nationals trying to leave southern Lebanon were now in a "safe zone".
It added that only a dozen people had decided to remain in the region.
The last 20 Swiss to be evacuated out of the country are expected to arrive in Switzerland on Saturday.