Powerful aftershocks continued to rattle the badly-hit Aceh region on the northern tip of Sumatra island, where 104,000 people were killed last month and separately, key aid donor Germany hinted it would offer more debt relief.

The US navy Seahawk helicopter crashed down in a paddy field just outside Banda Aceh, the nerve centre of a huge international relief operation, shortly after 7:15 am (0015 GMT), injuring at least four of the 10 people on board.

Witnesses described smoke pouring from the engine of the aircraft, from the US navy's San Diego-based HS2 anti-submarine squadron, shortly before it hit the ground.

The helicopter had been flying in from the aircraft carrier USS Abraham Lincoln, which has been moored off Sumatra for more than a week, dispatching a fleet of 17 helicopters on missions to deliver aid to isolated communities.

Lieutenant Commander John Daniels told AFP by telephone from the Lincoln that there were no serious injuries among the crew and all remaining helicopters were continuing their missions.

The crash, the cause of which was not yet known, was the latest in a series of hitches to blight delivery of relief to the hungry and sick along Aceh's coastal wastelands.

Despite millions of dollars of aid pouring into Aceh, logistical problems including bottlenecks at Indonesian airports have caused serious delays in distributing supplies.

Last Tuesday Banda Aceh's airport was closed for most of the day when a Boeing 737 cargo plane skidded off the runway after swerving to avoid a buffalo.

The United Nations said Sunday although aid was reaching most areas of the region, some people were still without help, while the outbreak of disease remained a threat to people gathered in relief camps.

UN officials in the west Aceh town of Meulaboh, which was almost completely destroyed when a magnitude-9.0 quake sent waves crashing inland on December 26, said they were rushing in vaccinations after confirming a measles case.

Also in Meulaboh, Indonesia's military claimed separatist rebels were hampering relief efforts, posing a security risk as foreign aid workers attempted to deliver supplies.

Colonel Geerhan Lantara, army chief for west Aceh, said the country was justified in maintaining a state of emergency imposed in the region before the disaster as part of efforts to crush a decades-old rebellion.

"The emergency has to remain. If not the rebels will be a threat to the masses," Lantara said, adding that in one incident, the rebels had attempted to stop locals getting clean water in a town to the north of Meulaboh.

Despite a shooting incident in Banda Aceh in the early hours of Sunday -- attributed to a stressed-out Indonesian soldier firing in the air -- the United Nations said it did not believe there were any dangers to foreign aid workers.

Nerves were tested again early Monday however, when a powerful tremor measuring 6.2 on the Richter scale shook Banda Aceh and surrounding areas for several minutes. There were no reports of further damage or casualties.

Meanwhile in Jakarta, visiting German Foreign Minister Joschka Fischer indicated his country and others may be willing to cancel some of Indonesia's foreign debt to help it overcome the disaster.

Fischer, who met with Indonesian President Susilo Bambang Yudhoyono, said in addition to a moratorium on debt repayments agreed by the Paris Club of creditor nations, Germany was ready to "go further".

Germany is one of the biggest foreign aid donors for tsunami victims, stumping up 654 million dollars ahead of a major summit in Jakarta last week at which the UN appealed for an urgent one billion dollars in cash.

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