Gunfire and explosions crackle in the hills. Plumes of smoke from burning villages streak the monsoon-grey sky. Refugees fleeing for their lives are pouring into Bangladesh over the Myanmar border as the conflict between Myanmar security forces and Rohingya militias escalates and risks spiralling into a humanitarian disaster.

The refugees say their villages are being raided and burned. They tell stories of the indiscriminate killing of civilians at the hands of security forces and Buddhist nationalists. Since 25 August, more than 18,500 Rohingya, a largely Muslim ethnic group, have fled into Bangladesh from Rakhine state. However, UN sources say they believe the true figure is closer to 28,000. And Bangladeshi aid workers claimed on Saturday that 70,000 – almost 10% of the Rohingya population – had crossed in less than 24 hours. “This is a new dimension,” said Adil Sakhawat, a journalist with the Dhaka Tribune.

Dozens of rickety shelters dot the fields surrounding the town of Gundum, where Rohingya refugees first started arriving last week. Goats and chickens that were brought across by the refugees dart between the muddy feet of old women and small children soaked by the monsoon rains.

Rusting machetes, arranged in expectation of the sacrificial cow slaughter for Eid al-Adha, are propped against bamboo sticks supporting a tarpaulin. Bengalis walk along the road with the odd cow that a refugee has sold them. Myanmar cattle are widely seen as the best quality and are in high demand across Bangladesh.

On top of those already here, UN sources have said there could be another 20,000 refugees stranded in a narrow strip of no man’s land that separates mainly Buddhist Myanmar and predominantly Muslim Bangladesh. Prevented from entering Bangladesh by border guards, these Rohingya have limited access to relief and have been forced to rely on the help of local villagers to survive.

Rakhine state and the Rohingya are no strangers to ethnic violence. Fighting last October forced 87,000 Rohingya into Bangladesh. However, both refugees and Sakhawat said this year was different. “There were large influxes in 2012, 2015, and 2016, but this time it has broken every record,” he said. “It is drastically different.” Ominously, there are now fewer men entering Bangladesh than in previous years. Sakhawat, who walked four hours into the mountains in the Baichari area of the border, said: “The Rohingya told me that the Myanmar military are indiscriminately killing men of fighting age.” He said he also heard accounts from four villages in northern Maungdaw that “the military are taking the children from the arms of their mothers, and throwing them away”.

Fortify Rights, an NGO that is documenting human rights abuses in Rakhine, reported that survivors from the village of Chut Pyin, in Sittwe district, spoke of the Myanmar army and local civilians beheading Rohingya. Survivors estimated that the death toll in the village, which once had a population of 1,400, was more than 200.

The Rohingya are from Myanmar’s western Rakhine state. Numbering about 1.1 million, there are now estimated to be more than 400,000 Rohingya refugees in Bangladesh. They face systematic discrimination in Myanmar and are often referred to as the world’s most persecuted minority. Myanmar denies them citizenship, alleging they are illegal immigrants from Bangladesh.

Myanmar security forces have started to place mines along the fence. Ultimately, [the refugees] will become trapped Bangladeshi border guard

This escalation in violence in Rakhine began when 30 Myanmar border posts and an army base were attacked last month by the Arakan Rohingya Salvation Army (Arsa), an insurgent group that began operating in October 2016. In response, security forces launched a “clearance operation”, supported by Buddhist militias that Rohingya refugees say are intent on driving them out of the country.

In Tumbru, the men of the Border Guards Bangladesh (BGB) look across at the makeshift refugee shelters squeezed into the no man’s land between the Naf river and the Myanmar border fence. Ringed by paddy fields and low hills, the monsoon has turned the ground into a sea of mud. A concrete Myanmar police outpost, nestled in the trees, overlooks the camp.

Smoke from nearby villages that have been torched is sometimes visible, and local villagers describe the buzzing of helicopters as Myanmar soldiers are moved between remote outposts and other Rohingya villages are raided.

One BGB soldier spoke for his unit as he grumbled that because of the surge in violence, “everybody loses”. This weekend’s Eid holidays have been cancelled for the guards. They are under orders to stop the Rohingya entering Bangladesh. The Bangladeshi government is keen to cultivate ties with Myanmar and fears that more refugees could disrupt its own campaign against militant Islam.

Every 200 yards, BGB soldiers stand watch under black umbrellas. Ankle deep in mud at some points, they blow their whistles and shout at Rohingya trying to swim across the river to go back.

On the far bank, a man jumps into the river with a mobile phone balanced in a plastic bag on his head. He is accosted by a guard on the other side. He has been sent to go to villages on the Bangladeshi side to ask for dry clothes. “I left my home five days ago,” said the panting man, whose name was Muhammad. “My son was killed by the Myanmar border police. He was two years old. He was called Mahmud.”

A BGB guard, who refused to give his name, said: “Myanmar is scared of Arsa. They don’t attack the villages where they know Arsa has a presence. They only hit villages where they know there are only civilians.” He said the refugees had told him “they are targeting civilians”.

Ashraful Azad, professor of international relations at Chittagong University, said the aim was ethnic cleansing. “Myanmar wants to remove all the Rohingya. It is genocide.”

Jahir Khan Babby, 19, lives in the village of Tombro, 200 yards inside Bangladesh. “Rohingya come here to ask for help,” he said. “We are giving them so much help.” Babby said that villagers, for a small sum, help anywhere between 50 and 100 Rohingya cross the river in the dead of night. “But if I was a Rohingya, I would fight for my land,” he said, echoing a common view among local Bangladeshis, who are quietly supportive of Arsa.

Hungry Rohingya refugees wait for food at the Kutupalong border camp in Bangladesh. Photograph: STR/AFP/Getty Images

Culturally, the Rohingya are similar to the Bangladeshis. Many who have entered Bangladesh since 25 August have relatives who fled there in previous years and have now helped smuggle them across. In Kutupalong refugee camp, many new arrivals have found shelter with friends and relatives.

Bangladeshis have also been coming to Tumbru to donate food. Nurul Absar, 16, is from Cox’s Bazar, 25 miles away. He and his family of seven came with a dozen plastic bags filled with bread, popped rice, oil and a large box of 7 Up. “We don’t have relatives here, but we are Bangladeshis,” he said. “They need help”.

The BGB soldiers turn a blind eye to the Rohingya children who swim back and forth, ferrying donations across the river and going to the nearby villages to ask for help. The soldiers said that every night the Myanmar border police came down to the fence to observe the camp. “These past two nights, they have started to place mines along the fence,” one soldier said. “Ultimately, [the refugees] will become trapped.”

But the dynamics of the conflict in Rakhine are changing. Arsa is growing in strength. Buoyed by greater support among the population they have embedded themselves in the shelters at night and keep watch over the border. Refugees say Arsa is receiving money and weapons from groups in Malaysia, Pakistan, Afghanistan and Australia, and Arsa fighters have reportedly said new attacks in Myanmar are imminent.

The Rohingya say that with greater support they could begin to push back against the Myanmar military. They are hoping for Bangladesh’s support, but this is unlikely to come. “The government is linking Arsa with home-grown Islamic networks,” said Azad. “They see Arsa as a threat and want to establish good relations with Myanmar at any cost.” Bangladesh recently proposed joint strikes and patrols.

Kutupalong refugee camp is where most Rohingya are aiming for. Here, those who fled as far back as a decade ago have established sturdier shacks capable of protecting them from the rains. Formally, there are 35,000 here but in reality the number is far higher. New arrivals, many with bruises and scars from the journey, mill around the camp entrance.

Muhammad Amin, 18, is hobbling by the side of the road. “The Myanmar military shot me in the leg when I was farming,” he said. “I came to Bangladesh on a stretcher.” There is no news from the village where he comes from as “there is nobody left there”. Besides him, an old man describes bluntly how his village was raided.

Fortify Rights has said it believes that a “massive influx” of Rohingya is likely to attempt to cross into Bangladesh in the coming days. Sakhawat said that in only five hours in the mountains, he saw more than 1,500. Many who have done so recently, like Aji Rahman, who had strung his possessions over his shoulder on a bamboo stick, resorted to perilous journeys through dense mountain forests. “I took five days to get here,” he said. “The military were shooting and running. My brother was killed by a shot to the head.” He gestured that the bullet passed clean through.

Nearby, is Halima Khatun, who is in her 20s. She is being rushed to the Medecins Sans Frontieres hospital at Kutupalong on a makeshift stretcher. She has just given birth and the newborn girl is being cradled by Khatun’s mother, who is walking a few paces behind.

In a Hindu village a mile north-west of Kutupalong, Hindu Rohingya from Rakhine are sheltering having also fled there through the mountains. The Observer counted about 200 people, mostly women, children and the elderly, crammed into a wooden barn.

“We fled because we heard the fighting and we heard that Hindus elsewhere had been killed,” a woman with three children said.

Azad said: “The problem is that nobody can show any convincing solution. The international community is limited to criticism. The Rohingya have no friends in Myanmar.”