Washington, Aug 18 - US President Barack Obama said the mission in Afghanistan was 'a war of necessity' as Al Qaeda was still plotting against the US.

However, he said the Iraq mission -- which he has repeatedly criticised as 'a war of choice' -- will end in 2011.

Addressing the Veterans of Foreign Wars in Phoenix, Arizona, Monday, Obama said that in Afghanistan the US was pitted against an enemy that would not be easily defeated.

'We must never forget. This is not a war of choice. This is a war of necessity,' he said of the Afghan mission.

The president added: 'Those who attacked America on 9/11 are plotting to do so again.