A sudden surge of refugees arriving in Calais is stoking new tensions between migrant communities and led to the mass brawl and multiple shootings that shocked France last week.

There has been a 25% increase in the number of migrants heading for the French port, placing pressure on food handouts and increasing competition for routes into the UK.

Migrants and charities blamed the rapid increase on a recent border treaty between France and the UK, which raised “false hopes” that it would be easier to reach Britain.

The treaty, signed by President Emmanuel Macron and Theresa May a fortnight ago, promised to accelerate the processing of migrants in the port city and was heralded as “a more humane approach”.

Yet while the new migrants had hoped for a quicker journey across the Channel, they discovered that there was little food and almost no shelter at the port.

Shortly after 2.30pm on Thursday at least five migrants were shot at close range as they queued for food handouts, prompting a running brawl between Afghans and Eritreans across the city. Four Eritreans aged between 16 and 18 remain in a critical condition in hospital as the French police hunt for the gunman, who is believed to be a 37-year-old Afghan people-smuggler.

Speaking in the city, close to where the shootings took place, Laura Griffiths, of the refugee charity Safe Passage, said: “The treaty led to many new arrivals but also rumours – false hopes – that there would be an easy passage to the UK. These new arrivals increased tension at distribution points. In reality, the only legal passage is for minors, and even this is not working effectively.”

Griffiths said that following the border treaty announcement the charity recorded the arrival of 200 migrants entering Calais from surrounding locations in northern France and Paris, bringing the total within the port to around 800.

François Guennoc, vice-president of the Calais charity L’Auberge des Migrants, said the UK-France border agreement had caused chaos. “It gave people hope to reach England,” he said. “People arrived suddenly, about 200, mainly underage people and women who arrived in Calais because they thought that the Home Office said they [could] go directly to England. Then they thought the Home Office was lying. People were upset – it was crazy.”

Facebook Twitter Pinterest A video grab of clashes that broke out in Calais ferry port after the shootings on 1 February. Photograph: Nord Littoral/via AP

Migrants said that the treaty had provoked frustration among desperate individuals, many of whom sleep rough in the woods near Calais.

Ifa Derrec, a 22-year-old ethnic Oromo fleeing violence in Ethiopia, said: “Many new persons arrived, and those already here became angry because they have been waiting a long time and then new people take their food. This causes the problem. We are too cold and hungry, there is violence because of this,” he said, shivering outside his flimsy tent on a patch of muddy wasteland.

Others volunteer alternative theories for the shooting, which has raised fresh scrutiny on how France intends to deal with the increasing numbers of migrants converging on Calais.

Ife Magiso, 20, from Ethiopia, described how a growing enmity between Afghan and Eritrean migrants in Calais began during a dispute over access to a lorry park an hour’s walk from the east side of the city. The lorry park, off the region’s main artery, the A16, offers the frequent chance of climbing into the backs of lorries headed to the port of Calais, but was controlled by Afghan smugglers, who refused to allow Eritreans to use it.

“They say it’s theirs. That’s unfair because we share our [lorry] parks with anyone,” said Heeran, 19, another Oromo migrant, who arrived in Calais three months ago.

Some accuse the Afghans of operating a shamelessly racist policy over access to the coveted truck stops outside the port. “They don’t allow any black people to go there,” said Abdul Wadood, 24, from Khartoum, Sudan.

A number of those gathered beside a row of tents pitched outside a warehouse near the Rue Garennes, a 10-minute walk from the old “jungle” camp – which held almost 10,000 migrants before it was demolished 15 months ago – said that resentment had been brewing between Afghan and Eritrean factions following an alleged incident shortly after the start of the year.

According to Rashad Mamoun, a 27-year-old from Sudan, a young Eritrean attempted to enter the lorry park claimed by the Afghan smugglers. “So they took his clothes, shoes, his phone, everything,” said Mamoun. “They left him naked, he had to walk back freezing. People are not happy with the Afghans, maybe the Afghans got scared, that’s why they shoot.”

But the use of a firearm in the long-running turf war among groups of different nationalities hoping to reach the UK has shocked many in Calais. One Afghan, 20-year-old Amir, from Herat, who has camped near the port since September, said: “I really hope there is no more shooting. Why has he not been caught?”

Yet reprisals are likely, the Eritreans say. “There will be revenge,” said Magiso, nodding solemnly. “But we only carry stones to throw, and that’s no good against guns.”

His friends said they had armed themselves with sticks, and within an hour of Thursday’s shootings a group of between 150 and 200 Eritreans carrying iron bars and sticks clashed with Afghans. “I will fight, but I don’t want to get shot,” said Eritrean Eyob Mebrahtu, 23, pointing to his thigh, where he said he had been stabbed but would not reveal by whom.

Not all are keen to join the conflict. Mohammed Montaset, 25, from Khartoum, who has spent six months sleeping in a copse, is terrified of the prospect of violence. “I have travelled here to find peace, for a better life. Now, every night, I sleep in fear.”

Four migrants in critical condition after Calais brawl Read more

His friend Omar, 19, from Kassala, Sudan, glanced towards his right arm, strapped in a sling. The teenager explained that he had been beaten with a stick while sleeping. “I don’t know who did it, why or which nationality,” he said.

Already the shooting has brought a crackdown, with security forces drafted into the city. Rumours have long persisted that the many people-smugglers operating in northern France had guns, with some Kurdish traffickers further east along the coast at Dunkirk alleged to have access to AK-47s, possibly from Syria. “We had heard they had weapons. Now we have confirmation,” said Guennoc.