A video showing what appear to be American forces in Afghanistan urinating on dead Taliban fighters could set back efforts to broker peace talks just as the Obama administration is launching a fresh round of shuttle diplomacy.

The video, which was 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-U.S. 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.

The administration of U.S. President Barack Obama, seeing a glimmer of hope in its effort to broker talks, is launching a fresh round of shuttle diplomacy with an immediate goal of sealing agreement for Taliban insurgents to open a political office in the Gulf state of Qatar.

Marc Grossman, Obama's special representative for Afghanistan and Pakistan, begins a diplomatic blitz this weekend that includes talks with Afghan President Hamid Karzai in Kabul and top officials in Turkey, Saudi Arabia and the United Arab Emirates.

The video will not help his efforts to build confidence among the warring parties.

Such action will leave a very, very bad impact on peace efforts, said Arsala Rahmani, the top negotiator from Karzai's High Peace Council.

Looking at such action, the Taliban can easily recruit young people and tell them that their country has been attacked by Christians and Jews and they must defend it, he said in the first comments from a high-ranking Afghan.

The U.S. 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 highly reprehensible and disgusting.

The behaviour depicted in this video is reprehensible and is not in keeping with the values of U.S. Armed Forces, ISAF spokesman Lieutneant Colonel Jimmie Cummings said.

ILLEGAL, AGAINST HUMANITY

News of the footage had yet to reach the streets of Kabul on Thursday but Afghans who were told about what the tape appears to show were horrified.

According to Islam and international law it's against humanity and it's illegal, said Sayed Abdul Samad, an elderly man with a long white beard and a white turban.

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.

Like several other city residents, Qaisullah, a 44-year-old shop keeper near the capital's Shah-e-dushamshera mosque, called for the soldiers in the video to be punished.

The government has to discuss with the U.S. government the prosecution of those soldiers so in future nobody will be able to make fun of Afghans, he said.

I haven't seen the video but I'd say it will harm our country and peace talks. It starts just with footage on TV but will end up with demonstrations around the country and maybe the world, Qaisullah said.

Karzai's office declined immediate comment and Taliban spokesmen were not available for comment.

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 U.S. 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.

Almost a year of behind-the-scenes efforts by U.S. negotiators appear to be bearing initial fruit as the Taliban comes close to taking steps toward what U.S. officials hope might become authentic talks on Afghanistan's political future.

In Kabul, Grossman will seek approval from Karzai -- whose support for a U.S. 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 goal is to move the talks beyond mostly logistical discussions of mutual confidence-building measures.

We are trying to get from conversations about confidence-building measures to negotiations between Afghans and the Taliban, said a senior administration official in Washington who declined to be identified.

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 extrication from a long and costly war.

The United States and its allies aim to withdraw combat troops by the end of 2014.

(Additional reporting by Phil Stewart, Missy Ryan and Warren Strobel in WASHINGTON; Writing by Robert Birsel; Editing by Paul Tait)