By Terry Friel

KABUL, Feb 4 (Reuters) The United States, which has just doubled its combat troops in Afghanistan, takes over command of the 33,000-strong NATO force in the country today amid warnings of a bloody spring offensive by the Taliban.

Outgoing commander British General David Richards, who in his nine months in charge saw his force grow from 9,000 and push into the Taliban heartland in the south for the first time, said 2006 had been a crunch year for the rebels and they had failed.

''Because last year they really did see that they had an opportunity to defeat NATO. As I've said, we foiled that attempt and much more,'' Richards said yesterday.

''The Taliban do talk about a spring offensive. I won't use that term because all they offer is more death, destruction and despair, against the vision of hope and growing prosperity of the government and the international community.'' Last year was the bloodiest since US-led forces toppled the Taliban in 2001 after the September 11 attacks in the United States.

More than 4,000 people, a quarter of them civilians and 170 foreign soldiers, died, mainly in fighting in the south and east, bordering Pakistan, where the Taliban and other militants have safe havens and training grounds.

MORE US TROOPS Late last month, the United States extended the tours of duty of some of its soldiers, effectively boosting troop levels by 2,500 for the next few months, which US ambassador to Afghanistan Ronald E Neumann says is almost a doubling of combat forces on the ground.

Washington has been pressing its allies for more troops, and for an end to restrictions some countries impose on how and where their soldiers can fight.

But so far, only Britain and Poland have committed more men and women and France is pulling its special forces out.

The extra US troops will help provide a rapid reaction force that Richards and other commanders constantly pushed for but were never given.

The Taliban warned last weekend that 2007 would be ''the bloodiest year for foreign troops'', saying they had 2,000 suicide bombers ready for a spring offensive when the winter snows melt in the next few months.

Last year saw a massive jump in suicide bombings, previously almost unheard of here, as militants copied tactics from Iraq.

Analysts warn there are still not enough troops in Afghanistan, saying attention has been distracted by Iraq and this will be the decisive year for the battle here.

''From the beginning, the United States did not put sufficient forces in Afghanistan in order to prevent a counter-insurgency from resurging,'' international security expert and professor of international relations at the US Ohio Wesleyan University Sean Kay told Reuters.

''NATO continues to suffer from this -- there are simply not enough troops to carry on a successful counter-insurgency campaign in the south.

''As the Taliban get further entrenched, the public there gets further drawn into their grip. And when we don't have enough troops to accomplish the mission, those that are taking risks do not have adequate reinforcements other than heavy airpower, which when applied doesn't exactly fit into a successful hearts and minds strategy.'' REUTERS AKJ HT1142