"The situation has spiraled into the world's fastest developing refugee emergency, a humanitarian and human rights nightmare," Guterres said in a speech to the UN Security Council.
More than 500,000 Rohingya refugees have flooded into neighboring Bangladesh. The exodus came after attacks by Rohingya militants on security posts prompted a Myanmar military crackdown last month.
The speech comes as witnesses and survivors claim at least 15 people, mostly children, drowned and others were missing when a boat carrying Rhongya refugees capsized off Bangladesh on Thursday.
"They drowned before our eyes. Minutes later, the waves washed the bodies to the beach," said Mohammad Sohel, a local shopkeeper.
The UN has received "bone-chilling accounts" of refugees being subject to "excessive violence and serious violations of human rights, including indiscriminate firing of weapons, the use of landmines against civilians and sexual violence," Guterres told the public session of the council.
"This is unacceptable and must end immediately," he added.
Myanmar's military has been accused of ethnic cleansing against Rohingya Muslims.
Guterres called on Myanmar to halt military operations, allow "unfettered access" for humanitarian aid, and the "safe, voluntary, dignified and sustainable return of the refugees to their areas of origin."
"The reality on the ground demands action -- swift action -- to protect people, alleviate suffering, prevent further instability, address the roots of the situation and forge, at long last, a durable solution," he said.
The UN chief noted that the "systemic violence" could cause unrest to spill into the central part of Myanmar's Rakhine state, threatening 250,000 Muslims with displacement.
Guterres said a donors' conference would be held on October 9, without specifying the location.
On Thursday night, witnesses and survivors said a boat carrying Rohingya refugeees overturned just yards from the coast in rough waters, after it was lashed by torrential rain and high winds.
United Nations Secretary General António Guterres address U.N. Security Council meeting on Myanmar's Rohingya crisis, Thursday Sept. 28, 2017 at U.N. Source: AAP
"They drowned before our eyes. Minutes later, the waves washed the bodies to the beach," said Mohammad Sohel, a local shopkeeper.
The tragedy is the latest in a series of deadly accidents as desperate refugees surge across the border into Bangladesh from neighbouring Myanmar, where the country's military has been accused of ethnic cleansing against Rohingya Muslims.
Distraught survivors
The latest drowning tragedy comes after a series of deadly accidents as desperate refugees surge across the border into Bangladesh from neighboring Myanmar.
Local police inspector Moahmmed Kai-Kislu told AFP 15 bodies including at least 10 children and four women had so far been washed ashore, and there were fears the number could rise still further.
The International Organization for Migration, which is leading the relief effort in the area, told AFP that one survivor said the boat sank as it tried to dock at a place that was out of sight of security forces.
"It's a very sad story. There were a hundred Rohingya on board when it sank," IOM spokesperson Hala Jaber told AFP.
"As (the captain) was trying to dock, the boat capsized and it was not far from the shore but it was far enough and was still deep," she said, adding that search efforts were ongoing.
One distraught survivor told AFP that his wife and one of their children had been killed when the ship sank.
"The boat hit something underground as it came close to the beach. Then it overturned," said Nurus Salam, who had set off set off for Bangladesh from a coastal village in Myanmar late Wednesday with his family.
Another survivor, who was weeping on the beach, told an AFP reporter that her parents and children were missing.
The UN's refugee agency (UNHCR) said 27 survivors had been located so far, including eight women and seven children.
'Desperate' escape
The huge influx of Rohingya to Bangladesh -- the largest mass movement of refugees in the region in decades -- was put at 501,800 by the UN Thursday.
The exodus began on August 25 when attacks by Rohingya militants on security posts prompted a Myanmar military crackdown.
It has created a humanitarian crisis as the government and aid agencies struggle to provide food, clean water and shelter.
Those who have made it to Bangladesh have brought with them harrowing accounts of murder and villages torched by Myanmar soldiers and mobs of ethnic Rakhine, who are Buddhists.
Rakhine, long a cauldron of ethnic and religious tensions, has been scarred by seething animosity since severe bloodshed erupted across the state in 2012.
The wave of violence has led to an outpouring of international criticism against the country's Nobel Peace Prize winning leader Aung San Suu Kyi, whose reputation as a human rights champion has been left battered.
But a diplomat at the UN said delegates were mindful she was treading a difficult line with the country's all-powerful army, which has led the military operations.
"We do not want to complicate civil-military relations in Myanmar," said a Western diplomat.
