The operation, named Lightning Resolve, is "kicking off as we speak," Lt. Gen. David Barno said in an interview at his headquarters in the Afghan capital.
He said the operation contains enough "offensive punch" to keep militants off-balance and would include targeted, intelligence-driven raids. He gave no specifics.
A 2,000-strong Marine force which has hammered Taliban militants in a southern stronghold since it arrived in March is in the process of leaving the country, Barno said.
The remaining force of 17,000 soldiers will intensify its cooperation with the United Nations, which is helping to organize the Oct. 7 presidential poll, seen as a key step on this war-shattered country's path to recovery.
U.S. forces are expected to provide a broad security blanket across the south and east during the election, leaving Afghan police and soldiers to protect polling stations.
NATO is expected to bulk up its 6,500-strong peacekeeping force focused on Kabul and fan out across the relatively peaceful north in the run-up to the vote.
The United Nations has helped register some 7 million voters — out of an estimated 10 million eligible people — despite a string of attacks that have raised fears security will not be adequate to ensure a fair and free vote.
"We'll be shifting our efforts to helping to build the required security going into the election itself," Barno said. "The specifics of that plan are still being worked out."
In the latest reported incident, dozens of suspected Taliban attacked a school used as a voter registration site in southern Kandahar province, shooting one election worker in the leg.
Three female election workers were fatally injured June 26 when a bomb hit their vehicle in the eastern city of Jalalabad. Taliban supporters claimed responsibility.
The day before, Taliban gunmen executed as many as 17 men in Uruzgan province after finding that they were carrying voter ID cards, Afghan officials said.
Because of the violence, registration has been uneven, with election teams still unable to enter parts of the Pashtun-dominated south.
Electoral officials have no census data to calculate the distribution of seats in parliament, and there are no laws yet on campaign finance or media access for 2,000 expected candidates.
The elections for president and parliament were originally supposed to occur in June, but were pushed back to September. Last week, the presidential vote was slated for October, while the vote for legislative offices was put off until next spring.
The United Nations is concerned that if the parliamentary vote is held too soon, anti-Taliban warlords who allied with the United States will consolidate their grip on the country after the failed drive to disarm them.
Only about a quarter of the militiamen supposed to disarm by the end of June have given in their weapons. No new deadlines have been set.
U.S.-backed interim President Hamid Karzai is expected to win the presidential vote, but he faces at least a half-dozen rivals in this ethnically and regionally fractured country. It is not clear whether he will garner the 50 percent majority needed for outright victory, meaning a run-off two weeks later may be necessary.
Incident-free elections would reflect well on the U.S. military and deflect criticism that it has failed to capture Osama bin Laden or Taliban supreme leader Mullah Mohammed Omar. The military is also dealing with a widening investigation into allegations that in American jails here.
Barno said the effort to track down top fugitives was "as robust as it's ever been here." But he said he still he has no firm intelligence on their whereabouts.
There have been reports from Washington that bin Laden and his top aide, Ayman al-Zawahri, might be planning major new al Qaeda attacks to disrupt the upcoming U.S. elections. But Barno said he had no knowledge of the men's whereabouts or what they might be up to.
"Those are, I think, the most difficult targets we have over here," Barno said. "Because of the lack of information we have on them, I think the inference is that they are well-protected."
He said U.S. intelligence-gatherers were hoping that Pakistani military operations against al Qaeda suspects in a tribal region across the border would yield vital leads.
Since it was consolidated in the 1700s, Afghanistan has never had a democratic government. It was ruled by monarchs — almost exclusively from the Pashtun tribe — for most of its history, although the last king, Mohammad Zahir Shah, did introduce a partially elected national assembly in 1964 reforms.
Nine years later, the monarchy was overthrown, but the republican government set up in its place soon collapsed. In 1978, a Marxist rebellion took power, follow the next year by the Soviet invasion and ten years of occupation. When the Soviets withdrew in 1989, a brief period of peace was shattered by civil war among several factions, including the Taliban.