WASHINGTON — The Pentagon has extended the tour of 2,200 Marines in Afghanistan, after insisting for months the unit would come home on time.

The 24th Marine Expeditionary Unit, which is doing combat operations in the volatile south, will stay an extra 30 days and come home in early November rather than October, Marine Col. David Lapan confirmed Thursday.

Military leaders as recently as Wednesday stressed the need for additional troops in Afghanistan. Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, has often praised the work of the 24th MEU in fighting Taliban militants in Helmand Province.

Defense Secretary Robert Gates, however, has repeatedly said he did not intend to extend or replace the U.S. Marines in Afghanistan, calling their deployment there an extraordinary, one-time effort to help tamp down the increasing violence in the south.

Asked about the possibility of an extension in early May, Gates said he would “be loathe to do that.” He added that “no one has suggested even the possibility of extending that rotation.”

Lapan said Thursday that commanders in Afghanistan asked that the Marines stay longer.

Pentagon press secretary Geoff Morrell said the longer tour does not open the door to an extension beyond the 30 days, nor to the possibility of replacing them with other U.S. troops when they come out in November. “This is a slight addition to this tour and nothing more,” he said.

He added that commanders in Afghanistan “asked for 30 more days to milk the fighting season to the bitter end and cement the gains they have made in the south.”

The Pentagon announced in January that the Marine Expeditionary Unit, which is based at Camp Lejeune, N.C., was being ordered to Afghanistan, largely because efforts to press other NATO nations to increase their troop levels at the time had failed.

At the same time, about 1,000 members of the 2nd Battalion, 7th Marine Regiment, which is based at Twentynine Palms, Calif., was ordered to deploy also. That unit has been used to train Afghan security forces. As a result of the MEU’s extended deployment, Marines from both units are now expected to return home at about the same time.

Commanders faced with increasing violence have said they need at least 7,500 more troops in Afghanistan. And President Bush and defense officials have said they hope to identify additional units by the end of the year that could go to Afghanistan early next year.

The Pentagon has said that more U.S. forces cannot be sent to the Afghan fight until decisions are made to further reduce troop levels in Iraq. In the last two months, violence in Afghanistan has led to more U.S. and coalition casualties there than in Iraq, and June was the deadliest month for U.S. troops in Afghanistan since the war began.

“The Taliban and their supporters have, without question, grown more effective and more aggressive in recent weeks ... as the casualty figures clearly demonstrate,” Mullen told a Pentagon press conference Wednesday.

The heavy fighting has claimed the lives of a dozen members of the Marine units. One other Marine’s death was not related to combat.

“It’s a very complex problem, and it’s tied to the drug trade, a faltering economy and, as I’ve said many times, the porous border region with Pakistan,” said Mullen. “There’s no easy solution, and there will be no quick fix.”

There are 32,000 U.S. forces in Afghanistan, including 14,000 serving with the NATO-led coalition and another 18,000 conducting training and counterinsurgency.

The NATO force includes more than 52,000 troops from as many as 40 countries.