Dean Yates, Banda Aceh – As the battered truck pulled up after negotiating some of Aceh's dangerous country roads, 20 traders crowded around, eager to get their supplies of tomatoes, chillis and dried crackers.

With their shelves half empty, deliveries from the fertile hinterlands of the Indonesian province of Aceh are welcome but increasingly rare as a government military offensive against Free Aceh Movement (GAM) rebels moves into its second week.

"Many drivers are just too scared and fuel is more expensive," said trader Basri Jalil, adding he had been forced to hike the price of tomatoes to 12,000 rupiah ($1.40) a kg from 2,000 a week ago.

Across this sprawl of stalls behind the imposing 125-year Baiturrahman Mosque, the spiritual heart of the local capital Banda Aceh, traders and shoppers told similar stories. "People are still buying, they just don't buy as much. They have to eat," said Jalil, before hauling off his stock.

Indonesian officials have sought to reassure people, saying steps are being taken to distribute staples in war-hit Aceh, under martial law as government forces stage their biggest ever offensive against GAM since fighting broke out 27 years ago.

But the United Nations has warned of a humanitarian crisis. One local official said food prices had risen across almost all of Aceh, home to four million people, because trucks were unable to deliver supplies. He said it was also too dangerous to allow relief agencies to distribute food in the countryside.

There have been attacks on traffic and vehicles set ablaze, blamed by the military on GAM. Fear of such attacks and getting caught in the crossfire has been too much for many drivers.

The owner of one grocery store here said he feared the food shortages could become the worst since the conflict began. "No goods at all are making it. See this, it's all old," he said, pointing to flaccid vegetables hanging from the ceiling.

Chief Social Welfare Minister Yusuf Kalla said on Sunday that trucks carrying food supplies would be protected. He also said the government would charter ships that could hold up to 60 trucks at a time to drop supplies to Banda Aceh and the city of Lhokseumawe, near some of the heaviest fighting.

Traders and residents said the worst shortages in Banda Aceh, 1,700 km northwest of Jakarta, were of vegetables, cooking oil, milk powder and cigarettes. But, crucially, they said rice supplies had not been hit.

Aceh's war has often brought the economy to a standstill and badly hit farming in one of Indonesia's most fertile places. But finding supplies is not the only problem. There are also Indonesian soldiers, long accused of operating with impunity in the resource-rich province, to deal with.

One fruit seller said three times in the past two weeks troops had dropped by asking for mangoes and apples and refused to pay. "I want to close my stall but I have debts," he said, begging that neither his name nor the stall's location be disclosed.