"My appeal to all the neighboring countries is to say I understand your security concerns, but please keep your borders open because these people are really fleeing for their lives," UNHCR chief Philippo Grandi said Tuesday during a visit to Nairobi to mark World Refugee Day.
Grandi also warned in an interview with AFP that the two-month-old war threatened to spread insecurity in the "fragile" nations that border Sudan.
"It is a worrying situation. Many of these neighboring countries are very fragile and there is also an element of insecurity that risks spreading," Grandi said.
At an earlier press conference in the Kenyan capital, Grandi said that half a million refugees had fled Sudan since the beginning of the conflict, and another two million were internally displaced.
"If we don't silence those guns, the exodus of Sudanese people will continue," said the United Nations High Commissioner for Refugees.
He was speaking a day after donors at a U.N. conference pledged close to $1.5 billion to combat the humanitarian crisis in Sudan and help its neighbours host those fleeing the fighting -- including Chad, South Sudan and Egypt.
That figure was however less than half of what humanitarians have said is required this year and Grandi appealed to the international community to dig deeper, saying donations were a fraction of defense spending by rich nations.
"I'm not saying that military spending isn't necessary, that's not my field and I understand the logic, but humanitarian aid is a tiny, tiny fraction of all that," he said.
"I can't believe that we can't make a bit more of an effort," he added, urging Gulf states in particular to do more.
"We are unfortunately gradually seeing the destruction of this country," Grandi told AFP, echoing remarks made by U.N. Secretary General Antonio Guterres at Monday's donor conference.
A record 25 million people - more than half of Sudan's population - now depend on humanitarian aid, the United Nations says.
A 72-hour ceasefire agreed by the warring sides took effect on Sunday to allow access to aid, although a number of previous truces have collapsed.
"It is a worrying situation because we have not seen much progress, if any, in the negotiations between the two generals who are fighting in the country," Grandi said.
"This must stop because it risks having incalculable consequences in the region and beyond."