Fighting across southern Afghanistan has left 28 suspected Taliban rebels dead, officials said Monday, amid stepped up violence in the countdown to crucial legislative elections next month.

Meanwhile, U.S. Marines and Afghan Special Forces in eastern Kunar province (search) Monday pushed deeper into Korengal Valley (search), which is controlled by militants suspected of ambushing a team of U.S. commandos and shooting down a special forces helicopter on June 28.

U.S. military spokesman Col. James Yonts said the forces were making progress in the operation, but cautioned it may take a long time.

"That area as we all know has historically been a safe haven for enemy forces. It is no secret that they still remain there," he told reporters in Kabul. "So what you see there is the continued aggression of the American forces taking the fight to the enemies of Afghanistan to rid them from that area."

The U.S.-led coalition, Afghan police and army are on the offensive across the country to prevent the Taliban and other militants from disrupting legislative elections on Sept. 18.

The bloodiest battle between Afghan forces and the Taliban occurred Sunday in Zabul province (search) when Afghan forces attacked a group of suspected militants, killing 16 of them and arresting one, the Defense Ministry said in a statement.

Among the dead was a local Taliban commander, Mullah Nasir, it said.

Also Sunday, in neighboring Uruzgan province's Dehrawud district, a gunbattle between Afghan soldiers and insurgents left five militants dead, the ministry said.

In Zabul, alleged insurgents mistakenly detonated a mine that was intended to hit a convoy of U.S.-led coalition and Afghan forces Sunday, killing one militant and wounding another, Sori district chief Rovi Khan said.

Then in an adjacent district, Tirin Kot, police hunted down and killed six suspected guerrillas who attacked a highway checkpoint, provincial Gov. Jan Mohammed Khan said. Nine alleged militants were also arrested in a sweep of the area.

No security forces were hurt in any of the clashes, according to the statement and governor.

Nearly 1,000 people have died in violence since March. Officials have warned of further unrest leading up to the polls, which are seen as a major step toward democracy in Afghanistan after more than two decades of war and civil strife.