Iran's president told the leaders of Aghanistan and Tajikistan today that the three neighbours could provide a counterweight to Nato in Asia once foreign troops quit the region.
Mahmoud Ahmadinejad said the three countries had all thrown off foreign domination in recent decades and should strengthen economic and security ties and be independent of big powers.
At a summit of the three Persian-speaking countries - the fourth in two years - he said Tajikistan and Afghanistan had emerged from Soviet rule and occupation just as Iran had shaken off US influence after the 1979 Islamic revolution.
"Many don't find this desirable - three independent countries forming a powerful friendship in the region, and changing the current situation. None of them is happy about this," he said in part of the meeting which was televised.
Iran sees the United States, which has troops in Iraq to its west and Afghanistan to the east, as an enemy. Washington is suspicious of Tehran's regional ambitions and is determined to stop it getting a nuclear bomb, something Iran denies it wants.
Mr Ahmadinejad called on foreign troops to leave the region and allow the three nations to develop closer bonds.
"The fate of the three countries are knotted together in different ways and those who impose pressure on us from outside, and who are unwanted guests, should leave. Experience has shown they never work in our interest.
"Those who came in from Europe representing NATO, they want to put pressure on China, Russia and India and if they are confronted by three independent, empowered countries here, then that is an obstacle," he said.
Afghan president Hamid Karzai, who addressed the other leaders seated around a triangular table as "my dear brothers", has been trying to improve relations with Iran in recent months.
He welcomed Mr Ahmadinejad to Kabul in March and said he would host him and Tajikistan's president Imomali Rakhmon at the next trilateral meeting in the Afghan capital at an unspecified date.
The leaders today agreed to work for greater cooperation in economic, trade and transport matters. Shi'ite Iran has the potential to act as a spoiler in any U.S. attempt to reach a political settlement in Afghanistan, ratcheting up or down its alleged support for Taliban insurgents depending on the extent to which its distrust of the Sunni movement is outweighed by its anger with the United States.
Analysts say Mr Karzai is already worrying about an eventual American withdrawal from Afghanistan and trying to shore up his position while seeking an accommodation with Taliban insurgents.
When the Taliban were in power, from 1996 to 2001, Iran along with Russia and India supported the then-Northern Alliance against the Taliban.
India meanwhile has been trying to improve relations with Iran, in what is seen as part of its efforts to counter Pakistan's influence there.