But by early summer, the Army said attacks were averaging about a dozen per day.

In September, the number of attacks exceeded 20 on some days.

The attacks are killing an average of three to six American troops per week.

"The enemy has evolved," Lt Gen. Ricardo Sanchez, commander of coalition forces in Iraq, said in Baghdad at the end of last week.

"It is a little bit more lethal, little bit more complex, little bit more sophisticated and in some cases a little bit more tenacious".

Sanchez said coalition forces need to be prepared for more casualties and a long stay in Iraq.

The increase in resistance suggests that raids on rebels and their arms caches so far are failing to reduce the number of attacks against the US-led occupation.

Coalition aircraft have been fired on at least eight times in the past six weeks, though the attempts missed their targets.

Some of the guerrillas used assault weapons, which are mostly ineffective against helicopters or airplanes.

But there have been three attacks using missiles or large-calibre anti-aircraft guns.

- USA Today

World News Headlines Of The Last 48 Hours

•  India security chief resigns after Mumbai attacks
•  Religious clashes kills hundreds in Nigeria
•  Cheeky thief steals Thanksgiving dinner
•  Zim offers free graves as cholera spreads
•  Zim parties agree on constitutional amendment
•  Protesters wounded in Thailand explosions
•  Obama to nominate Clinton as Sec of State
•  Obama to nominate Clinton as Sec of State
•  Late workers, students get notes blaming NY subway
•  Obama as role model for black youth? Not so fast
•  Somali pirates reach deal on ship
•  Some patients confuse lives with reality shows
•  100 missing after boat accident