"I came here to see and listen," Mrs Ogata told reporters before leaving on a tour of two camps housing some of the nearly 215,000 refugees displaced by the conflict.

Mrs Ogata stumbled through the mud and ice, accompanied by Russian soldiers, at the Sputnik camp, home to 7,500 refugees, and the Severny camp, where 7,000 more live in old railway carriages.

Refugees complained of lack of food, medicine and shelter, with some parents saying they slept outdoors so that their children could find room in tents.

The BBC's Andrew Harding reports: "The capital Grozny is still under siege"

She described conditions as terrible and promised more international aid, including 200 tents which would be delivered next week.

She also called for better management of the aid effort, before leaving for talks with the President of Ingushetia, Ruslan Aushev.

President Aushev, a fierce critic of Moscow's policies, warned that "the number of refugees could soon be higher than Ingushetia's population, which is now about 340,000".

Mrs Ogata called for the aid effort to be stepped up.

"As far as managing the camps is concerned, because of this large influx, I think there is maybe some more need for stronger capacity, larger human resources to be brought in," she said.

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As Mrs Ogata toured one camp, the refugees begged for help and accused Russian troops of killing civilians.

Chechen fighters are holding out in Grozny

"There is no room for us. It's cold. The children are freezing. There's no food and we are all ill," refugees shouted.

Some of the displaced have taken shelter with families, but the rest are living in tents, railway carriages, or abandoned buildings. Temperatures overnight are falling to -10C.

Moscow has been criticised for denying sufficient aid to the camps, but has rejected talk of a humanitarian crisis.

Its seven-week campaign against separatist Islamist rebels in Chechnya has triggered the flood of refugees.

The issue was figuring prominently at an OSCE summit which opened in Istanbul on Thursday.

Fighting

During Mrs Ogata's tour, the boom of Russian artillery could be heard from across the border.

Russian soldiers patrol the streets of Gudermes

Russian troops entered the key western Chechen town of Achkhoi-Martan on Thursday morning "without firing a shot", Russia's NTV television reported.

It showed pictures of a Russian general addressing a crowd of hundreds gathered in a square in the centre of the town.

Russian forces are continuing to tighten the noose around the capital Grozny.

They are also reported to be advancing on Urus-Martan, 15km from Grozny, and the last rebel outpost guarding south-west routes into the capital.

People fleeing the area said bombs and rockets were methodically destroying the town.

Interfax news agency quoted officials at the headquarters of federal forces in the North Caucasus as saying that troops were advancing on the town of Achkhoi-Martan, 30km southwest of Grozny, and moving deeper into the Urus-Martan region further east.

The agency quoted Russia's Defence Ministry on Wednesday evening as saying the Islamic fighters were falling back into the more mountainous terrain of southern Chechnya, mining roads and buildings as they went.

Russian warplanes and artillery were continuing their bombardment of sites reportedly linked to the rebels across a broad swathe of Chechen territory.

The ministry said aircraft had destroyed eight rebel strongholds, seven vehicles, a radio station and a TV tower.

But Russian forces also encountered resistance. Two helicopters near the eastern city of Gudermes came under fire on Wednesday but escaped unscathed, military officials said.

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