Survivors have recalled how they escaped from Boko Haram fighters in the aftermath of last week's devastating attacks on two Nigerian towns.

The witnesses' accounts came to light on Thursday after Amnesty International released satellite images showing what it says is "indisputable and shocking evidence" of the scale of the attacks.

Before and after images of two neighbouring towns, Baga and Doro Gowon in northeast Nigeria taken on January 2 and 7, show the effect of the attacks which left more than 3,700 structures damaged or completely destroyed. 

Alaji Bukar, a young boy who fled Baga to Maiduguri, more than 150km south, told Al Jazeera that fighters caught families as they tried to escape.

"They separated the children from the adults who were asked to lie down. They were shot dead," he said.

"We were taken back to their camp. We escaped when they were busy burning homes."

Another survivor, Muhammad Umar, said Boko Haram fighters chased him for more than a week before he made it to Maiduguri, the capital of Borno state.  

"They followed us on motorcycles and trucks. They shot and hacked at us. We were trampling on dead bodies as we ran in the bush," he told Al Jazeera.

"We kept on for two days before we reached the main road and five to reach safety. We've lost more than 1,000 people. The whole town is burned down as far as I see."

President visiting

President Goodluck Jonathan made a surprise visit to Borno state on Thursday, his forst visit to the area worst affected by the advance of Boko Haram fighters, since he declared a state of emergency in the northeast in May 2013.

Jonathan has been criticised for not doing enough - and often not saying enough - about relentless Boko Haram attacks that have left thousands of civilians dead, hundreds kidnapped and pose the greatest security challenge to the country.

Daniel Eyre, Nigeria researcher for Amnesty International, said the detailed new images show "devastation of catastrophic proportions". 

Satellite image of the village of Doro Gowon in northeastern Nigeria taken on January 2. The red areas indicate healthy vegetation.

Last week, the human rights group noted reports of as many as 2,000 dead in the attacks, but the Nigerian military has so far cited a figure of 150 dead, including slain fighters. 

"Of all Boko Haram assaults analysed by Amnesty International, this is the largest and most destructive yet. It represents a deliberate attack on civilians whose homes, clinics and schools are now burnt out ruins," Eyre added.

Medical charity Doctors Without Borders said on Tuesday that its team in the Borno State capital, Maiduguri, was providing assistance to 5,000 survivors of the attack.

Satellite image taken on January 7, showing almost all of the structures razed and much of the vegetation cleared. The inset demonstrates the level of destruction of most structures in the town. 

US Secretary of State John Kerry said in Sofia, Bulgaria, on Thursday that he and British Foreign Secretary Philip Hammond were discussing a special initiative to deal with Boko Haram in Nigeria, but did not elaborate any further.

Kerry described the group as "one of the most evil and threatening terrorist entities on the planet".

Meanwhile, the Danish government allocated $2m to help the victims of Boko Haram. Mogens Jensen, the country's foreign aid minister, said on Thursday that Denmark was acting "in solidarity with the affected families" and wanted to give them clothes, food, shelters and "a possibility to get through the crisis".

Nigeria's home-grown Boko Haram group drew international condemnation when its fighters kidnapped 276 schoolgirls from a boarding school in northeast Chibok town last year. Dozens escaped but 219 remain missing.

Source: Al Jazeera and agencies