Niger has declared a 15-day state of emergency in the border region of Diffa after a spate of attacks by Nigerian militant Islamist group Boko Haram.
The declaration gives troops in Diffa new powers to search homes without a warrant and imposes a curfew.
A female suicide bomber was shot dead in Diffa town on Wednesday before she could detonate herself, an army spokesman told the BBC.
Boko Haram has been stepping up attacks on neighbouring states in recent weeks.
The group attacked Chadian army positions in north-eastern Nigeria on Wednesday but were repelled.
"We knew they were going to attack us," a Chadian military official told Reuters news agency. "We were waiting. The battle didn't last long."
The battle took place in Gamboru town in Nigeria's Borno state, which Chadian troops captured from Boko Haram last week.
Chadian forces are part of a regional coalition fighting Boko Haram, whose militants are increasingly attacking the area around Lake Chad - a crossroads between Nigeria, Chad, and Niger.
Thousands of residents are fleeing the town of Diffa, attempting to travel the 500km (310 miles) to Zinder city, reports the BBC's Baro Arzika from Niger.
One refugee told the BBC she had fled Diffa and was trying to locate her relatives.
"I left home when I heard the blast at Tattasai market and went looking for my children," she said.
"I left Diffa yesterday but I don't have peace of mind because I could not locate my mother, my son, my aunties and my other relatives.
"Everybody went their separate way and I did not meet any of them on my way here. I searched almost everywhere in Zinder but I couldn't find my son."
Another witness said that about 200 refugees had arrived in Zinder on Tuesday in a single convoy, travelling in lorries.
Niger has been targeted by Boko Haram over the past week, with residents reporting a car bomb in Diffa town and assaults by militants on nearby Bosso.
The state of emergency in Diffa gives the region's governor powers to introduce new restrictions without passing any laws.
Nigerian information minister Yahuza Sadisu Madobi told the BBC that the government would not use its emergency powers to conduct a house-to-house search and would only search houses under suspicion.
The curfew will force Diffa residents to stay indoors between 20:00 and 06:00 local time (19:00 and 05:00 GMT).
Approximately 3.2 million people in Nigeria have fled their homes as a result of Boko Haram's insurgency, Nigerian officials say.
Nigeria launched an investigation on Tuesday after reports of rapes, child trafficking and other abuses in camps for those displaced.
The conflict with Boko Haram has forced a postponement of Nigeria's presidential and parliamentary elections from 14 February to 28 March.
The group says it is fighting to create an Islamic state.
Boko Haram attacks in 2015
- 14 January: Nigerian military repel attack on the town of Biu in the north-eastern state of Borno
- 18 January: Suicide bomber kills four people after detonating a car bomb at a bus station in north-eastern Yobe state
- 25 January: Militants attack strategically important north-eastern city of Maiduguri, with dozens reported killed
- 4 February: Militants kill up to 70 people in attack on Cameroon
- 6-8 February: Attacks on Niger repelled by Niger's military
- 9 February: Bus hijacked at up to 20 kidnapped in attacks on border areas of Cameroon