KABUL - A video showing what appear to be American forces urinating on dead Taliban fighters prompted anger in Afghanistan and promises of a US investigation on Thursday, but the insurgent group said it would not harm nascent efforts to broker peace talks.

The video, posted on YouTube and other websites, shows four men in camouflage Marine combat uniforms urinating on three corpses. One of them jokes: "Have a nice day, buddy." ?Another makes a lewd joke. ?

It is likely to stir up already strong anti-US sentiment in Afghanistan after a decade of a war that has seen other cases of abuse, and that could complicate efforts to promote reconciliation as foreign troops gradually withdraw.

"Such action will leave a very, very bad impact on peace efforts," Arsala Rahmani, a senior member of the Afghan government's High Peace Council, told Reuters.

The administration of US President Barack Obama, seeing a glimmer of hope after months of efforts to broker talks, is launching a fresh round of shuttle diplomacy this weekend.

Marc Grossman, Obama's special representative for Afghanistan and Pakistan, will fly into the region for talks with Afghan President Hamid Karzai and top officials in Turkey, Saudi Arabia and the United Arab Emirates. ?

His immediate goal is to seal agreement for the Taliban to open a political office in the Gulf state of Qatar.

Despite concerns when the video first emerged that it would not help his efforts build confidence among the warring parties, a Taliban spokesman said although the images were shocking, the tape would not affect talks or a mooted prisoner release.

"This is not the first time we see such brutality," said spokesman Zabihullah Mujahid.

"We know that our country is occupied...This is not a political process, so the video will not harm our talks and prisoner exchange because they are at the preliminary stage."

The US Marine Corps has said it will investigate. The NATO-led International Security Assistance Force (ISAF) in Afghanistan described the acts depicted in the video as "disrespectful" and "inexplicable".

"This behaviour dishonours the sacrifices and core values of every service member representing the 50 nations of the coalition," ISAF said in a statement, adding that a US criminal investigation had been launched.

Karzai's office declined immediate comment. ? ?

"ILLEGAL, AGAINST HUMANITY"

News of the footage had yet to spread in Afghanistan -- a country where a minority have access to electricity and the Internet is limited to an tiny urban elite -- but Afghans who were told about what the tape appears to show were horrified.

"The Afghan government has to discuss with the US government how to prosecute these soldiers, so in the future nobody will be able to make a joke of Afghans," said middle-aged merchant Qaisullah, who has a shop near the Kabul's Shah-e-dushamshera mosque.

"It may start with just video footage, but it will end with demonstrations around the country and maybe the world."

Anti-American feeling has boiled over, or been whipped up, into violence several times in Afghanistan in recent years. Protests over reports of the desecration of the Muslim holy book have twice sparked deadly riots.

"They've committed a crime. We don't want them in our country," said Feda Mohammad, a middle-aged man in jeans, a jacket and a woollen hat, when told about the tape. "We don't like foreigners in our country and they have to leave."

DEEPLY TROUBLED

In the United States, two military officials, speaking on condition of anonymity, said the video appeared to be authentic, but Reuters could not verify it or its source independently. ? ?

The US military has been prosecuting soldiers from the Army's 5th Stryker Brigade on charges of murdering unarmed Afghan civilians while deployed in Kandahar province in 2010. ?

At the Pentagon, Captain John Kirby said the defense department was "deeply troubled" by the video, which was released at a critical time for what US officials hope might become authentic talks on Afghanistan's political future. ? ?

In Kabul, Grossman will seek approval from Karzai -- whose support for a US effort he fears will sideline his government has wavered -- to move ahead with a series of good-faith measures seen as an essential precursor to negotiations that could give the Taliban a shared role in governing Afghanistan. ?

The diplomatic initiative includes a possible transfer of Taliban prisoners from the Guantanamo Bay prison.

A breakthrough would mark a milestone for the Obama administration, struggling to secure a modicum of stability in Afghanistan as it presses ahead with its gradual withdrawal from a long and costly war. The United States and its allies aim to withdraw combat troops by the end of 2014.