Concern is growing for the safety of people in the conflict area
The Sri Lankan military says it has designated a safe zone for civilians as it pushes ahead with its offensive against Tamil Tigers in the north-east.
Military spokesman Brig Udaya Nanayakkara said advancing troops would not fire into a 32 sq km buffer zone.
"We have already air dropped leaflets to let local people know, and are also passing on the message via the International Red Cross," he said.
Aid agencies say there are about 250,000 civilians in rebel-held areas.
Separately, the UK Foreign Office said Prime Minister Gordon Brown had written to Sri Lankan President Mahinda Rajapaksa to express concerns about the situation in the country.
The BBC's Ethirajan Anbarasan in Colombo says the army's announcement comes a few days after reports from medical staff inside rebel-held territory said that at least 18 civilians had been killed in recent fighting.
The Sri Lankan government has strongly denied the claims and criticised the BBC and other news outlets for broadcasting them over the weekend.
The announcement of the buffer zone came as the army continued its offensive into the Tigers' sole remaining stronghold in the north - the area surrounding the town of Mullaitivu.
The ICRC has appealed for the dead to be taken to Vavuniya
The military say that the safe zone will be on the A-35 main road which links Paranthan and Mullaitivu. The rebels have made no comment yet on the safe zone.
Our correspondent says that the government announced a similar zone a few months ago but those areas have now been captured.
Both human rights groups and aid agencies have raised concerns over the safety of people living in the conflict area.
The International Committee of the Red Cross (ICRC) spokeswoman in Geneva, Carla Haddard, told the BBC Sinhala service on Tuesday that the priority was to evacuate wounded and ill people in the north to hospital in the town of Vavuniya.
She said that so far this had not been possible. Nor had it been possible, she said, to get aid and medical equipment to the north because the ICRC had not been given security guarantees by either the government or the Tamil Tigers.
"The civilian population could be at great risk if we cannot resume safe passage and the movement of aid," she said. "This is one of our big concerns."
Ms Haddard said that the ICRC also faced difficulties in organising the transfer of bodies.
"What is happening on the spot is that the storage capacity of the morgue in Vavuniya hospital is reaching its limit so the hospital is burying its own bodies," she said.
'Move the war'
The campaign group, Human Rights Watch, recently accused the rebels of preventing people from fleeing the conflict area.
The army says it is closing in on the rebels
But the Tamil Tigers say that they are protecting civilians who voluntarily move with them.
Correspondents say the rebels are also trying to move the war to the east.
Officials on Wednesday said that at least two people, including a policeman, were killed in a bomb attack in Batticaloa.
Also on Wednesday, the UK expressed concern at the humanitarian situation, recent attacks on journalists and human rights abuses.
In a written statement, Foreign Secretary David Miliband said Sri Lanka had a "responsibility to safeguard the rights of all its citizens and adequately to address their political concerns".
He said: "Our consistent position remains that for peace to be sustainable, an inclusive political process that takes fully into account the legitimate concerns of all Sri Lankan communities - Sinhalese, Tamil and Muslim - is essential."
The Tamil Tigers have been fighting for a separate homeland for 25 years. At least 70,000 people have been killed in the insurgency.
The rebels had established a de facto state squeezed between government-controlled Jaffna in the north and the rest of the country.
But the latest military offensive has forced the rebels to give up much of their territory.
The military is on high alert to prevent the possible escape of top Tamil Tiger leader Velupillai Prabhakaran, although some analysts say it is unlikely he will be captured alive.