Afghanistan's main opposition bloc on Sunday called for presidential polls to be held next April, as laid out in the constitution, rather than in September as scheduled by the election organisers.

The elections will be a major test for President Hamid Karzai, whose popularity is waning seven years after U.S.-led and Afghan forces toppled the Taliban regime.

The Taliban insurgency has now spread from traditional militant strongholds in the south and east to areas just outside the capital, Kabul.

"The National Front wants the upcoming election to be held on time and we don't want an illegitimate president," former President Burhanuddin Rabbani, leader of the main opposition block, told a news conference.

"Any other dates will be against the constitution. We are against any changes conflicting with the Afghan constitution."

The Afghan constitution says the elections are to be held between 30 to 60 days before the presidential term is up, meaning they should take place around April next year.

But Afghanistan's Independent Election Commission says an agreement was reached between the government and opposition parties, including the National Front, to hold the polls later in the year because of the difficulty of organising elections during the harsh Afghan winter.

The National Front is a coalition made up mostly of parties led by former warlords that led the Northern Alliance that helped U.S.-led forces topple the Taliban government in 2001.

Although Karzai has not said for sure that he will run for a second term in office, he has strongly hinted that he will.

No other prominent candidate has come forward either and the National Front is unlikely to garner enough support to win the presidency as it is mostly made up of parties representing Afghanistan's ethnic Tajik, Uzbek and Hazara minorities from the north rather than traditionally dominant Pashtun majority from the south and east.