UK charities have launched an emergency appeal to save lives in Sudan's Darfur region and neighbouring countries.
The Disasters Emergency Committee (DEC) says 4.5m people are affected by the conflict in the region while looming rain threatens to bring further misery.
Hundreds of thousands have been living in camps for up to four years.
DEC head Brendan Gormley said: "We are seeing one of the greatest concentrations of human suffering right now in Darfur and Chad".
Some 200,000 people have died in Darfur since a rebellion began in 2003.
Pro-government Arab militias have been accused of widespread atrocities, such as mass killings, rape and looting black African villages.
The DEC, which represents 13 aid agencies, says the start of the rainy season brings the risk of conditions such as diarrhoea and malaria which threaten children, pregnant women and the elderly in particular.
It says malnutrition levels are already rising in some areas and "vital help" is needed to respond to a rapidly-growing crisis.
The DEC said aid agencies also needed to bolster life-saving food and medicines before the rains hit anytime in the next four weeks.
The money raised by the appeal will help provide shelter, clean water, sanitation and emergency food.
It will also help medical teams to provide emergency care and buy items such as water buckets and blankets.
Mr Gormley said: "The money raised by the British public in 2004 saved thousands of lives and we desperately need your help again.
"We have been keeping people alive but access is already severely hampered by conflict and the rains will make it much harder for us to respond if we don't act now.
Sudan's army is accused of backing Arab militias in Darfur
He said the fact the charities had come together to appeal showed the "severity of the situation".
"Charities are working tirelessly in the region. With your help, they can literally save thousands more lives. We need to act urgently - please do give generously."
The DEC said violence in the region was escalating and many villages were "burnt out shells".
The conflict has left two-thirds of the population in Darfur dependent on aid, it said.
Rebel group took up arms in 2003, accusing Sudan's central government of ignoring the region.
Sudan denies charges that it then mobilised Arab militias, known as the Janjaweed, to punish Darfur's civilians.
The US says a genocide is being committed in Darfur but Sudan says the scale of the suffering has been exaggerated.
The violence has spilled over the border into Chad, which has a similar ethnic make-up to Darfur, and the Central African Republic.
To donate to the Darfur and Chad Crisis Appeal call 0870 60 60 900 or visit www.dec.org.uk.