Insurgent attacks in Afghanistan rose by 11 percent in the past three months over the same period last year, according to the latest figures released by NATO's US-led coalition.

The month of June alone accounted for the highest number of attacks in nearly two years, with more than 3,000 assaults, including firefights and the explosion of homemade bombs, the International Security Assistance Force said.

The upturn comes as the US-led coalition starts to withdraw its 130,000 troops after more than 10 years of war and ahead of a 2014 deadline for an end to combat operations.

ISAF says one reason for the increase was an earlier start to the summer fighting season because of an early end to the harvest of opium poppies -- a major source of income for Taliban Islamist insurgents.

It gave another as the increased presence on the battlefield of Afghan security forces and better reporting of attacks against them.

"Enemy-initiated attacks over the last three months (April-June) are 11 percent higher compared to the same quarter last year," ISAF said in a report on its website.

"The annual start of the poppy harvest period is characterised by a considerable decrease in enemy-initiated attacks usually followed by a few weeks of lower attack levels.

"This year's harvest started later and finished earlier in the most poppy prevalent areas of Afghanistan compared to last year."

Despite the increase in attacks, the number of coalition deaths in the first six months this year -- 220 -- was down on the same period last year when 282 died, according to figures kept by the website icasualties.org.

About half of all deaths in both periods were due to roadside bombs, the statistics show.

"Insurgents continue to rely on IEDs (improvised explosive devices) as the principal means to execute their campaign," ISAF said.

In the latest attack, two NATO soldiers were killed on Thursday in an IED explosion in southern Afghanistan, ISAF said in a separate statement, without giving further details.