The defence secretary has told MPs the success of military operations in Afghanistan should not be measured by the number of body bags sent back to the UK.

In a Commons statement, Geoff Hoon said that the troop deployment, which ended on Monday, had proved effective in keeping al Qaeda at bay.

"This was a major operation, the largest offensive ground operation UK forces have undertaken since the Gulf conflict," he told MPs.

Admitting that there had been no contact with an enemy, the defence secretary said it would be "quite wrong" to portray the mission as a failure.

"The success of Operation Snipe should not be measured by the number of dead al Qaeda and Taliban fighters. No terrorists were killed or detained by the Royal Marines. There were no close contacts with the enemy.

"But to say that this means the operation was a failure, or a waste of time, as some have suggested, is quite wrong. One obvious and tangible proof of the operation's success was the destruction of a massive arms cache," he said.

Hoon rejected reports that British troops had only succeeded in destroying munitions of supporters of the interim administration. "This cache belonged to al Qaeda or the Taliban and not to a local warlord," he insisted.

But there was also a warning that "al Qaeda and the Taliban have not completely disappeared".

"The terrorists have not been finally defeated. Elements of these organisations continue to pose a threat inside Afghanistan. The enemy still exists. We need to keep up the pressure and prevent them from re-establishing themselves in Afghanistan."

Commenting on the news that eight troops had fallen ill in Bagram, Hoon said the situation was "serious".

One soldier has already been returned to the UK, with a further six returning home today. And about 350 soldiers have been quarantined at a medical hospital in Bagram while investigations are carried out into the cause of the illness.

"This is clearly a very serious situation. We are, however, encouraged that there have been no new cases over the past 22 hours," Hoon said.

"The precise nature of the illness is not yet known and medical tests are being urgently conducted to isolate the cause. The symptoms are consistent with enteric febrile illness and the illness appears to be contagious."

Shadow defence secretary Bernard Jenkin said there would be "questions to answer" about the outbreak.

But he gave his backing to the deployment of British forces for Operation Snipe. He said better intelligence would be the key to success in what was becoming a counter-insurgency war.

The defence secretary also told MPs that Turkey was on course to take over leadership of the International Security Assistance Force in Kabul by the end of June.

Geoff Hoon had earlier updated the cabinet, where he said that "people should not judge the campaign in Afghanistan by the marines and others by the number of body bags".