Nearly 125,000 mostly Rohingya refugees have entered Bangladesh since a fresh upsurge of violence in Myanmar on August 25, the United Nations said on Tuesday, as fears grow of a humanitarian crisis in the overstretched camps.

The UN said 123,600 had crossed the border in the past 11 days from Myanmar's violence-wracked Rakhine state.

Their arrival has raised fears of a fresh humanitarian disaster as already crowded camps in Bangladesh -- home to around 400,000 Rohingya refugees before the latest crisis -- struggle to cope with the influx.

Many are sleeping in the open air and are in dire need of food and water after walking for days to reach safety, the UN's main coordinator in Bangladesh said in a report.

"There is an urgent need for emergency shelters and for land to build these shelters on," said Vivian Tan, spokeswoman for the UN refugee agency.

"These people have walked for days. Some of them haven't eaten for days since they left. They survived on rain water and ground water."

Bangladesh initially tried to block the refugees, stepping up border patrols and pushing some back into Myanmar.

But in recent days they appear to have largely given up trying to prevent an influx that Prime Minister Sheikh Hasina said Tuesday was a "big burden" for Bangladesh.

- 'Humanitarian crisis' -

Thousands lined a major highway near the border, blocking traffic, while others crammed into small shops and huts to shelter from the monsoon rains.

"We walked for three days to reach Bangladesh... Both (my daughters) are now suffering from high fever and coughing," Rozina Khatun, a Rohingya from Myanmar, told AFP.

At a nearby clinic run by Doctors without Borders, scores of people lined up with bullet wounds and deep scars apparently left by machetes.

Bangladeshi rights campaigner Nur Khan Liton said a "massive humanitarian crisis" was unfolding.

"People are staying in refugee camps, on the roads, school yards and under open sky. They are clearing forest to create new settlements. There is an acute crisis of water and food," he told AFP.

The latest unrest broke out when a Rohingya militant group launched a series of coordinated ambushes on Myanmar security posts in response to what it said was a fresh crackdown.

The Muslim Rohingya are seen as illegal immigrants in mainly Buddhist Myanmar and have suffered decades of persecution, according to rights groups.

Unverifiable testimony from those who have fled has alleged tit-for-tat mass killings and villages being torched by the army, Buddhist mobs and Rohingya militants.

At least 11,000 Rakhine Buddhists and Hindus have also fled arson and attacks by militants to camps inside Myanmar, according to the last government update. 

- Landmines -

Bangladesh border officials say those fleeing also face the risk of landmines on the frontier.

On Tuesday, two Rohingya children were injured by an apparent landmine blast as they tried to flee unrest in Myanmar, border guard commander Manzurul Hasan Khan told AFP.

"They stepped onto some sort of explosives this morning and one of them lost his leg," Khan said.

A Rohingya woman had a leg blown off in the same area on Monday and guards said two more explosions were heard later Tuesday. All three have been taken to hospitals in the Bangladeshi border town of Cox's Bazar.

It is not known what caused the blasts, but Human Rights Watch said it feared that there could be more landmines in the area.

Khan said many Rohingya were also entering Bangladesh with bullet wounds, although it was impossible to say how these were sustained as media access to the worst-hit parts of Myanmar's neighbouring Rakhine is limited.

Rakhine has been a crucible of religious violence since 2012, when riots erupted. Scores of Rohingya were killed and tens of thousands of people -- most of them from the Muslim minority -- were forced into displacement camps.

But the current round of fighting, which broke out when Rohingya militants ambushed security installations, is the worst yet.

Myanmar's army has said nearly 400 people have died in the fighting that ensued, including 370 Rohingya militants.

Source: AFP