KABUL, Afghanistan - Afghan police and soldiers manned checkpoints at almost every intersection Thursday, searching vehicles and frisking their occupants in a massive security operation ahead of elections to choose a new president to guide the country after international combat forces withdraw.

Insurgents fighting the Western-backed government have intensified attacks ahead of Saturday's runoff vote, and the Taliban issued a new statement warning citizens to stay away from the polls. The first round in April passed relatively peacefully, but a recent assassination attempt against one of the two candidates left in the race has stoked fears of more violence.

"The Islamic Emirate deems it necessary to alert the people and warn them for the last time that they should not participate in this American process, deliberately or inadvertently," the Taliban said in a statement posted online.

Still, the senior U.N. envoy for Afghanistan expressed confidence Afghan voters would turn out as they did in the first round to decide their future by picking a new leader to oversee the transition after most U.S. and allied forces pull out by the end of this year.

Jan Kubis, the U.N. secretary-general's special representative for Afghanistan, also called on the candidates to give electoral authorities time to tally the ballots - most of which will come from remote regions, often transported by donkeys - and resolve any complaints amid widespread fears of fraud.

"Give a chance to due process, respect the work of the Commissions, don't jump to conclusions," he said. "Don't make statements or comments in anticipation of the results. it will just mislead the people. control yourself, act as responsible politicians."

He was referring to the likelihood that the campaigns of front-runner Abdullah Abdullah, the target of last week's attack, and former Finance Minister Ashraf Ghani Ahmadzai will start releasing their internal tallies before formal results are announced.

The official timetable is for preliminary results to be announced on July 2 and final results on July 22 in order to allow time for ballots to be secured and fraud complaints investigated.

The stakes are high as the winner will replace President Hamid Karzai, a one-time U.S. ally whose relations with Washington have soured, in the first peaceful democratic transfer of power in the country's history.

Karzai has governed Afghanistan since the Taliban was ousted following the U.S. invasion in 2001, and is constitutionally barred from seeking a third term.

Karzai on Thursday called on Afghans to vote for the candidate of their choice in order to bring about a "stable, secure and developed future."

The Obama administration is watching closely. Both candidates have pledged to sign a security pact with the U.S. that would allow thousands of international forces to stay in Afghanistan in a largely training and advisory capacity. Karzai has refused to sign it.