COX’S BAZAR, Bangladesh (Reuters) - More than 18,000 Rohingya Muslims, many sick and some with bullet wounds, have fled the worst violence to grip northwest Myanmar in at least five years, while thousands more are stuck at the Bangladesh border or scrambling to reach it.

Friday’s series of coordinated attacks by Rohingya insurgents on security forces in the north of Myanmar’s Rakhine state and ensuing clashes triggered the Rohingya exodus, while the government evacuated thousands of Rakhine Buddhists.

Since the attacks, about 18,445 Rohingya - mostly women and children - have registered in Bangladesh, the International Organization for Migration (IOM) said on Wednesday.

“They are in a very, very desperate condition,” said Sanjukta Sahany, who runs the IOM office in the southern town of Cox’s Bazar near the border.

“The biggest needs are food, health services and they need shelter. They need at least some cover, some roofs over their heads.”

Sahany said many crossed “with bullet injuries and burn injuries,” and that aid workers reported that some refugees “gave a blank look” when questioned.

“People are traumatized, which is quite visible.”

The United Nations, while condemning the militant attacks, has pressured Myanmar to protect civilian lives without discrimination and appealed to Bangladesh to admit those fleeing the military counteroffensive.

At least 109 people have been killed in the clashes with insurgents, Myanmar says, most of them militants but also members of the security forces and civilians.

The United Nations Security Council was briefed behind closed doors on Wednesday on the escalating violence at the request of Britain.

“There’s a lot of concern about the situation in the country. We all condemned the violence, we all called on all the parties to de-escalate,” British U.N. Ambassador Matthew Rycroft told reporters after the briefing.

The treatment of about 1.1 million Muslim Rohingya in Myanmar is the biggest challenge facing national leader Aung San Suu Kyi, who has been accused by Western critics of not speaking out for a minority that has long complained of persecution.

Rycroft said the Security Council looks to Suu Kyi to “to set the right tone and to find the compromises and the de-escalation necessary in order to resolve the conflict.”

The Rohingya are denied citizenship in Myanmar and regarded as illegal immigrants, despite claiming roots that date back centuries.

The violence marks a dramatic escalation of a conflict that has simmered since October, when a similar, but much smaller, series of Rohingya attacks on security posts prompted a fierce military response, in which the U.N. has said security forces probably committed crimes against humanity.

“The situation is very terrifying, houses are burning, all the people ran away from their homes, parents and children were divided, some were lost, some are dead,” Abdullah, 25, a Rohingya from the region of Buthidaung, told Reuters, struggling to hold back tears.

Abdullah said four of the six hamlets in his village of Mee Chaung Zay had been burned down by security forces, prompting all its residents to flee toward Bangladesh.

Slideshow ( 7 images )

He was among the thousands of terrified people who left their village to gather at the foot of the Mayu mountain range.

Together with his wife and 5-year-old daughter, Abdullah brought sticky rice, fetched plastic sheets and empty water bottles, preparing to trek in monsoon rain for days on a 20-km (12-mile) route through the mountains to the border.

“I am waiting for all of my relatives to leave together with my family as soon as possible,” he added.

Slideshow ( 7 images )

EXPLOSIONS AND FIGHTING

Myanmar officials have said the country had the right to defend itself from attack, adding that security personnel were told to keep innocent civilians from harm.

Mine explosions and fighting continued, the government said, blaming Rohingya militants for burning down houses and fleeing to the mountains after attacks.

Bangladesh is already host to more than 400,000 Rohingya refugees who have fled Buddhist-majority Myanmar since the early 1990s.

Dhaka has asked the U.N. to pressure Myanmar over its treatment of the Muslim minority, saying it cannot take any more.

At least 4,000 people were stranded in no man’s land between the two countries, with temporary shelters stretching for several hundred meters on a narrow strip between the Naf River and Myanmar’s border fence.

On Tuesday, Reuters reporters saw women, some carrying children and sick people, wade through the river, which narrows to less than 10 meters (11 yards) there. Bangladeshi border guards allowed groups of about six to cross to reach a stack of donated medicines.

Many Rohingya trying to cross were sick and at least six died after crossing over, one aid worker said, adding that some refused to seek help for fear of being caught and sent back.

Shaheen Abdur Rahman, a doctor at a hospital in Cox’s Bazar, said 15 people admitted since last week had gunshot wounds, varying from grazes to bleeding in the lung. Four serious cases were sent for treatment to nearby Chittagong.

Injuries also included fractures that could have been suffered in beatings or accidental falls while fleeing, he said.

“We don’t discriminate,” said Rahman. “Everyone coming to this hospital, whether they’re Bangladeshi or not from Bangladesh, we provide due service to them.”

In Kuala Lumpur, capital of neighboring Muslim-majority Malaysia, police said they arrested about 155 of roughly 1,200 mostly Rohingya demonstrators who protested against the renewed violence.

Tens of thousands of Rohingya have fled to Malaysia over the years, few with valid travel or identity papers.

(For a graphic on Rohingya conflict click tmsnrt.rs/2gjxOdv)