By Andrew Gray

WASHINGTON, July 3 (Reuters) - Some 2,200 U.S. Marines battling insurgents in southern Afghanistan have had their tour of duty extended by 30 days, U.S. officials said on Thursday.

The move comes amid U.S. concern about rising violence in Afghanistan from Taliban Islamist militants and other groups. The Marines' departure date was shifted from October into November, officials said.

Defense Secretary Robert Gates made the decision at the request of Army Gen. David McKiernan, commander of the NATO-led International Security Assistance Force in Afghanistan.

"The commanders are trying to milk the summer fighting season until the bitter end and trying to cement the significant gains the Marines have made in the south," Pentagon press secretary Geoff Morrell said.

The Marines belong to the 24th Marine Expeditionary Unit, which deployed earlier this year to boost NATO forces in southern Afghanistan, the scene of some of the worst insurgent violence.

June was the deadliest month for foreign forces in Afghanistan since the U.S.-led invasion in 2001 ended the Taliban's rule, with 42 troops killed in combat, according to a Reuters tally.

President George W. Bush acknowledged on Wednesday that June had been a "tough month" in Afghanistan.

Also on Wednesday, Adm. Mike Mullen, chairman of the U.S. military's Joint Chiefs of Staff said he was "deeply troubled" by the violence.

"The Taliban and their supporters have, without question, grown more effective and more aggressive in recent weeks, and as the casualty figures clearly demonstrate," Mullen told reporters.

Bush's opponents have accused him of neglecting Afghanistan and focusing too much on the war in Iraq, now in its sixth year. Bush has rejected the criticism and U.S. officials have called on NATO allies to contribute more troops and other resources to Afghanistan.

U.S. officials had repeatedly said they had no plans to keep the Marines in Afghanistan for any longer than their seven-month deployment and would not provide troops to replace them at the end of the traditional summer fighting season.

Morrell, the Pentagon press secretary, said the extension was relatively slight and there would be no further changes.

"This does not in any way open the door to a longer extension or to the prospect of backfilling the Marines," he said.

The United States has some 32,000 troops in Afghanistan -- around 14,000 in the NATO-led force and some 18,000 performing missions from counter-terrorism to training Afghan forces.

There are around 145,000 U.S. troops in Iraq. (Editing by John O'Callaghan)

AlertNet news is provided by