PRESIDENT George Bush yesterday gave the order to United States troops to lead an unprecedented international aid mission in Somalia under United Nations auspices, authorising them to take 'whatever military action is necessary' to speed food and medical supplies to the starving victims of drought and civil war.

The action will be spearheaded by 1,800 US Marines who arrived in Somali waters two days ago. They are due to make first landings by helicopter early next week, probably on Monday. Initial plans call for the gradual mobilisation of just over 28,000 US troops, who will be joined by soldiers from about 12 other countries, including France, Canada and Nigeria. France said it was sending between 1,500 and 2,000 troops. Britain is sending RAF transport aircraft to Somalia, but the Ministry of Defence has ruled out sending ground troops.

Responding to a UN Security Council resolution passed on Thursday, the President made the announcement in a national televised address, the first since his election defeat by Bill Clinton. In it, he stressed the limited role assigned to US troops, insisting they would be withdrawn as soon as the supply lines were secured. 'This is not an open-ended operation,' he said.

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The White House had already indicated that it would like the lead role of the US to be over before Mr Clinton's inauguration on 20 January. However, in a separate briefing yesterday the Defence Secretary, Dick Cheney, and the chief of staff, General Colin Powell, made it plain that US troops would almost certainly remain longer.

Mr Cheney warned against setting what he called an 'artificial deadline', arguing that the US troops would be withdrawn only when the mission was complete. He said he did not 'want to be bound' by the transfer of power on 20 January.

He was echoed by General Powell, who said his instinct was that it would be 'two to three months' before the US would be able to relinquish responsibility for the aid effort to UN peace-keepers. He added that while 28,000 was the initial estimate of US troops likely to be deployed, 'they may go up considerably, they may come down'.

General Powell said the first landings by the Marines already in the area would begin early next week. The Marines will start by securing the port and airport at Mogadishu and then move on to create safe conditions at the inland town of Baidoa. After that, probably about one week later, the first of the follow-up troops would start to arrive, he said.

The core of the US contingent will be 16,000 members of the 1st Expeditionary Marine Force from Camp Pendleton in California, who will be joined by 10,000 light infantrymen from the Fort Drum army base in New York State.

They will be airlifted to Somalia on board C-130 heavy-lift aircraft. It is planned that all the US troops should be in action in Somalia by the end of the month, and will begin to fan out over the whole of southern Somalia.

In the final phase, when aid distribution has been assured, the US troops would hand over responsibility for maintaining the aid effort to UN peace-keepers before beginning their withdrawal. A small contingent of US troops would probably stay behind, however, General Powell said.

The Pentagon meanwhile confirmed that it had already dispatched an aircraft carrier, the USS Ranger, and two other ships, a cruiser and a destroyer, to the area. The ships, carrying heavy firepower including 60 fighter aircraft, should be in position within a few days. They will stay in the region as contingency back-up but may not be used.

'It is now clear that military support is necessary to ensure the safe delivery of the food Somalis need to survive,' Mr Bush said, describing scenes of warehouses bulging with food in the Mogadishu docks while people were starving only a kilometre away. Naming the effort 'Operation Restore Hope', he added: 'We must help them live. We must give them hope.'

In a warning to the warlords in Somalia who have impeded UN efforts until now, Mr Bush underlined that the troops were authorised to use force to protect their lives and the lives of other Somalis. 'This is serious business,' he said. 'Our mission is humanitarian, but we will not tolerate armed gangs ripping off their own people.'

But the President, aware of concern both domestically and internationally over a long-term involvement of the US in Somalia, underlined the limited nature of American intentions. 'We will not stay one day longer than is necessary,' he said.

Mr Clinton endorsed the operation, calling it a 'historic and welcome step'. The President-elect said: 'Today President Bush has demonstrated that the American people will do their part to bring this tragic suffering to an end.'