Stray bullets whistled past Senegalese villager Modou Badjie and his family as they ran through a cashew forest in the middle of the night to reach neighbouring Gambia.

Badjie, his three wives, many children and extended family members are among more than 690 people who have crossed the border to escape a flare-up in fighting between soldiers and separatists in Senegal's southern Casamance region, according to government figures.

"Their shots could have hit one of us," Badjie said in the Gambian village of Upert, some 4 km (3 miles) from the border, where the family took shelter in March.

"People ran away because of the fighting and left all their belongings behind," he said. "We lost everything and are very tired."

The Senegalese military launched an operation on March 13 against rebels in Casamance fighting for independence.

The army said the operation was the clear the forest area where the rebels had camps which they used for illegal activities including growing marijuana.

Formed in 1982, the separatist movement has been largely dormant since a ceasefire in 2014 but continues to launch occasional attacks, prompting military interventions.

The latest clashes internally displaced over 6,000 villagers in both Senegal and Gambia, where households have been hosting refugees, placing pressure on the tiny West African nation of around 2 million people, the government said.

Badjie's dozen relatives moved in with a family of around 15 who offered to host them.

Women and children sleep together under the corrugated iron roof of their cement home on mattresses laid on the floor. The men stay in tarpaulin tents donated by the Gambian Red Cross.

"It is very hot inside," said Badjie, attempting to tie down loose sheets of canvas flapping in the wind. "We have to wait until midnight to go in."

Badjie's host Suleyman Sonko said humanitarian assistance had been slow and sparse.

"When food arrived (in May) it was not enough," Sonko said. "We decided to give all the rice to our guests."

Despite the discomfort, Badjie is too wary to venture back. Days after their arrival, a Gambian villager picking cashew nuts near the border was hit by a stray bullet.

"We want the military to go home so that everyone can be happy," said Badjie.