Parliament's five-year term expired in mid-2015 but polls were postponed amid concerns over whether the vote could be fair and transparent after a disputed presidential election in 2014.

President Ashraf Ghani extended the assembly's mandate until elections were feasible, prompting many to question the legality of his decision.

"After long consultations with concerned government bodies, we have decided to schedule the parliamentary election for July 7, 2018," said election commission chief Najibullah Ahmadzai.

He urged the government and international community to provide funding and support for the election, which will likely be held against the backdrop of a worsening security situation in Afghanistan.

"We have learnt lessons from the challenges and problems of the last elections. We are committed to holding (fair and transparent) elections," Ahmadzai said.

Both Ghani and his rival Abdullah Abdullah, a former anti-Taliban resistance fighter, claimed to have won the fraud-tainted 2014 presidential election, tipping Afghanistan into a crisis that threatened to trigger nationwide unrest.

It took months for both candidates to agree on a US-brokered deal to form a "national unity government".

Washington is soon expected to announce an increase in the US military deployment to bolster Afghan forces as they struggle to contain the Taliban insurgency. American military commanders in Afghanistan have requested thousands of extra boots on the ground.