Thousands of refugees who fled weekend fighting in the Chadian capital have arrived in Nigeria after a gruelling journey through Cameroon and are camping in the open in remote border towns.

In Gamboru-Ngala, a dusty outpost near Nigeria's northeastern border with Cameroon, a Reuters reporter saw hundreds of haggard refugees sitting in the shade of trees awaiting help. Many said they had not eaten for days.

Most were women and children and appeared to have left in a hurry as they had few possessions apart from the clothes they were wearing. Some had walked for days while others had paid to ride on open-top trucks.

"We were in N'Djamena when the fighting started. We heard gunfire from above and saw a helicopter spitting fire at the people. Many innocent people were killed," said Abdulkarim Abubakar, a young man who came with his mother and sister.

"Some of our relatives were killed and we ran away because we don't want to suffer the same fate," he said in Hausa, the lingua franca of northern Nigeria and parts of Chad.

A regional immigration official in the northeastern state of Borno said there were about 1,100 refugees from Chad in Gamboru-Ngala alone, with about 3,000 more expected to arrive from overstretched Cameroonian towns over the next few days.

The International Committee of the Red Cross (ICRC) in the Nigerian capital Abuja said Red Cross volunteers in Borno state had registered 2,423 refugees by Wednesday morning. The ICRC has provided the local Red Cross with emergency funds for basic rations of food and water.

Cameroon has borne the brunt of the refugee crisis caused by the fighting in N'Djamena. Fearing fresh attacks after two days of fierce battles between government troops and rebels who surrounded the presidential palace, more than 50,000 people fled south into northern Cameroon.

Most of the refugees who have carried on into Nigeria are Chadian but many are Nigerians who usually live in Chad. The borders between Chad, Cameroon and Nigeria are porous and many people move freely across them for work and family reasons.

Nigerian officials Visiting Gamboru-Ngala said they would assist the refugees with food and shelter. The state governor instructed local authorities on Tuesday to use their own funds to provide food for the refugees.