Kathy Marks, Banda Aceh – The group of men drinking coffee at a roadside stall sprang out of their chairs as the convoy of trucks and buses passed by in a cloud of dust. They had seen little traffic on the main highway through Indonesia's Aceh province since martial law was declared a week ago. "Bringing food, Insyaallah [God willing]," said one elderly man, placing his palms together.

With civil war raging between Indonesian troops and separatist guerrillas in Aceh, the lorry drivers who bring in staple goods from neighbouring North Sumatra are afraid to ply the route in case they get caught in the crossfire. Food supplies are running low across Aceh, which has a population of 4 million, and the United Nations agency Unicef has warned of a looming humanitarian crisis.

The convoy that left the provincial capital, Banda Aceh, yesterday to pick up supplies in Medan, the capital of North Sumatra, was escorted by a tank, an armoured police truck and an armoured car mounted with machine-guns, with dozens of troops deployed to protect it.

But armed escorts do not guarantee safe passage in these wild times. A separate convoy of 27 trucks that left Medan yesterday laden with food was ambushed by 10 masked rebels of the Free Aceh Movement (GAM) south of the city of Lhokseumawe. In the volatile district of Bireuen, a bus and a truck carrying vegetables were set on fire after their occupants had been ordered out.

The Indonesian government, which declared martial law in Aceh last week on the eve of the biggest military offensive against GAM, has been slow to address humanitarian needs. Local officials say a massive relief operation is required, particularly in the countryside, where dangerous conditions have prevented the distribution of food.

Jusuf Kalla, the Social Welfare Minister, has promised to send hundreds of trucks into Aceh in the coming days. But, in a move that dismayed aid organisations, he said that international relief agencies would be required to channel humanitarian aid through the government – to avoid the perception of outside interference in the conflict.

Banda Aceh, located on the far northern tip of Sumatra island, is experiencing some of the worst food shortages. The arrival of a battered truck at the city's open-air market yesterday sent a wave of excitement through the maze of stalls standing in the shadow of the imposing Baiturrahman Mosque. Two dozen traders crowded around, eager to secure their supplies of tomatoes, red chillies and dried crackers.

One trader, Yunan, said he had been forced to double the price of eggs and cooking oil, two commodities that are particularly scarce. Eggs now cost 800 rupiah each. Limited supplies are not his only concern. "I'm worried because there are so few shoppers," he said. "There is no transport bringing people into town from the countryside."

In rural areas, the plight of civilians grows more acute by the day. Health services have collapsed and the military says 15,000 people have been displaced, although no one – including the Indonesian Red Cross and the military – appears able to say where they are.

There are also chilling reports of plans to force thousands of people to leave villages in GAM stronghold areas and intern them in temporary camps to be built on sports fields. Military sweeps would then be launched and anyone left in those districts would be assumed to be GAM members and shot.

The security forces – which include regular TNI troops and Brimob military police – are frustrated by the difficulty they face in identifying rebels, who often blend in with the civilian population to avoid detection. All civilians in Aceh have now been ordered to apply for new identity papers signed by local police and military authorities.

In an attempt to scupper the scheme, rebels have been descending on villages and confiscating people's identity cards en masse. Villagers in Simpang Empat became the latest to suffer this fate after being raided by armed men yesterday.

The authorities are also using the cloak of martial law to crack down on human rights workers, whom they automatically regard as GAM sympathisers. Twenty activists were arrested last weekend in Banda Aceh, and 12 students suspected of GAM links were also detained. "We're all worried," said Effie, who works for Flower Aceh, an organisation that campaigns for women's rights.

And, despite Jakarta's insistence that the military operation in Aceh would be transparent and accountable, the media is being seriously obstructed in the province. Every day brings some new and absurd ruling from the military authorities, such as their demand that journalists stop quoting GAM leaders.

Journalists are being routinely prevented from entering villages where alleged military atrocities have taken place. A crew car from Metro TV, Indonesia's largest television network, was fired on when it tried to reach one settlement by a back road. The Independent was questioned for two hours at a military checkpoint and threatened with detention after visiting a village where five men had been shot dead.

The New York-based Committee to Protect Journalists sent a letter to Megawati Sukarnoputri, the Indonesian President, last weekend urging her to order the military to respect press freedoms.