WASHINGTON: Barack Obama is set to make the boldest strategic move of his presidency tomorrow and order a surge of tens of thousands more US troops into Afghanistan.

In front of cadets at the West Point military academy, the president is expected to announce between 30,000 and 35,000 reinforcements as part of a new strategy to "finish the job" there.

He is also under pressure to lay out an exit strategy in his 8pm address to the nation.

Many doubt the wisdom of escalating the conflict, and with more and more troops being deployed as casualty rates soar comparisons have been drawn with the Vietnam War that doomed Lyndon Johnson's presidency.

About 35,000 US soldiers were fighting the insurgency when Obama became president. After a boost in February, there are now about 68,000. The announcement could see troop levels triple under his tenure.

"You have to learn lessons from history. On the other hand, each historical moment is different.

"You never step into the same river twice. And so Afghanistan is not Vietnam," Obama said in September.

More than 900 US soldiers have died and October was the deadliest month since the start of the war with 74 soldiers killed.

Piling more pressure on a stretched US military, the surge will also expand the huge budget deficit, and Obama's Democratic allies fear the war's cost will suck hundreds of billions from vital projects at home.

Obama is also seeking more troops from reluctant Nato allies who will wait until after the London conference in January before taking a decision.

The Washington Post said up to 9,000 Marines will be deployed to the Helmand province, after Obama's address and some 1,000 army trainers will follow them, perhaps by February.