A conference meant to show governments would unite to support Afghanistan was overshadowed on Sunday by a row between Washington and Tehran after Iran said it had shot down a U.S. spy drone in its airspace.
Iranian threats of retaliation over the alleged intrusion added to a storm brewing in the region after Pakistan boycotted the conference in protest against NATO air strikes that killed 24 Pakistani soldiers on its border with Afghanistan on November 26.
International forces in Kabul said the drone may have been one lost last week while flying over western Afghanistan.
Iranian television quoted a military source as saying Tehran had shot down the drone in eastern Iran.
The Iranian military's response to the American spy drone's violation of our airspace will not be limited to Iran's borders, the military source said, without elaborating.
Iran has been accused in the past of providing low-level backing to the Taliban insurgency, and diplomats and analysts have suggested Tehran could ratchet up this support if it wanted to put serious pressure on U.S. forces in Afghanistan.
Tehran, at loggerheads with the United States over its nuclear programme, denies backing the Taliban.
The row over the U.S. drone is likely to add to despondency over the conference in the German city of Bonn, conceived to set Afghanistan on a path of long-term stability.
This is an unfortunate incident happening at the wrong time when the Bonn conference is being convened, an Afghan government official told Reuters. I hope this issue is not dragged into Afghanistan's event.
Afghan President Hamid Karzai was expected to use the conference to appeal for international financial and military support for a decade after combat troops withdraw.
Afghanistan will certainly need help for another 10 years, until around 2024 ... We will need training for our own troops. We will need equipment for the army and police, and help to set up state institutions, Karzai told Der Spiegel weekly.
If we lose this fight, we are threatened with a return to a situation like that before September 11, 2001, warned Karzai, referring to Taliban rule.
Yet with Western countries struggling to cope with economic downturn, the United States and European powers are arguing about who should foot the bill to keep Afghanistan afloat after most foreign combat troops leave in 2014.
A World Bank study released last month said Afghanistan was likely to need around $7 billion a year from the international community to help pay its security and other bills long after foreign troops leave.
In such an environment, and with Western public opinion already weary of the 10-year-old war in Afghanistan, the tensions between the United States and Afghanistan's neighbours Iran and Pakistan could not come at a worse time.
Only weeks ago, Turkey hosted a regional conference in Istanbul at which countries including rivals Pakistan and India, along with Russia, China and Iran pledged to work together to bring peace to Afghanistan.
Yet the NATO airstrikes on November 26 -- in circumstances which are fiercely contested by Washington and Islamabad - have plunged U.S.-Pakistan relations to a new low, and also worsened a difficult relationship between Afghanistan and Pakistan.
The United States and Afghanistan say leaders of the Afghan insurgency are based in Pakistan -- a charge it denies -- and want the Pakistanis to bring them to the table for peace talks.
The row with Iran tapped into a long-standing fear that its dispute with the West over its nuclear programme could come to a head at a time when the United States and its allies are struggling to extricate themselves from the war in Afghanistan.
Last Tuesday, Iranian protesters stormed the British embassy in Tehran after London announced sanctions on Iran's central bank in connection with its nuclear programme. Iran denies Western allegations it is seeking nuclear weapons.
Britain evacuated all diplomatic staff and closed its embassy, and also expelled Iranian diplomats from London.
The drone row underlined how far the dispute over Iran's nuclear programme could bleed into the Afghan war.
NATO's International Security Assistance Force in Afghanistan said in a statement the drone cited by Iran may be a U.S. unarmed reconnaissance aircraft that had been flying a mission over western Afghanistan late last week.
The operators of the UAV lost control of the aircraft and had been working to determine its status.
A U.S. official said there was no indication so far that the Iranian had shot down the drone.
Iranian Foreign Minister Ali Akbar Salehi held talks in Bonn with German Foreign Minister Guido Westerwelle on Sunday, but made no comment. A German foreign office spokesman said Westernwelle told Salehi the ransacking of the British embassy was perceived as an attack on all European Union members.
It was not clear whether Salehi would talk directly with U.S. Secretary Hillary Clinton during Monday's discussions.
The meeting in Bonn, former capital of West Germany, was being attended by foreign ministers from more than 80 countries and takes place a decade after a first Bonn conference on Afghanistan, which ended in high hopes for its future.
Westerwelle said the talks would focus on three areas: security, internal reconciliation and long-term support from world nations.
He also said he believed Pakistan still wanted stability in Afghanistan despite its boycott of the talks.
I have the impression Pakistan not only wants to cooperate in Afghanistan's stabilisation process but that it is in its own interests (to do so), he said in an interview with Deutschlandfunk radio.
(Additional reporting by David Brunnstrom, William Maclean and Missy Ryan)