Each time it seems that fraught ties between Washington and Kabul could not get worse, a new revelation proves the apparent nadir just another waypoint on the road to greater hostility.

The latest is a report that the Afghan president, Hamid Karzai, believes many insurgent-style attacks are masterminded by the US. The Washington Post quoted senior officials inside the presidential palace saying Karzai has kept a list of complex bombings across the country that he suspects were planned by Americans, although the officials also admit they have no firm evidence of a US role.

Topping his list of possible American plots is a recent suicide assault on a high-end restaurant in the Afghan capital, where three American citizens were among the 21 dead.

The idea that Washington is trying to violently undermine a government it has spent billions propping up, while carelessly taking the lives of its own citizens, provoked even the normally temperate US envoy to Kabul into a strong response.

"It's a deeply conspiratorial view that's divorced from reality," ambassador James Cunningham told the paper, adding that though he was aware of such allegations he had never heard them directly from the president.

"It flies in the face of logic and morality to think that we would aid the enemy we're trying to defeat."

Karzai's spokesman did not respond to requests for comment, but the official quoted by the Washington Post hinted that Karzai believed the US had staged attacks to draw attention away from its own civilian casualties. A history of disputes including attempts to remove him by skewing an election five years ago, have fed Karzai's antagonism towards the country that first brought him to power and still pays most Afghan civilian and military bills.

"The suspicion and the current environment of distrust is framed by the sad experiences of the past," the senior official was quoted as saying.

The relationship is no less poisonous on the other side, with one former ambassador on record as saying that Karzai needed medication for mental health problems. To many in America, the Afghan president's suspicion of a US role in suicide attacks was further evidence of a loose grip on reality.

"Every day and in every way, Karzai's strategic and tactical delusions get more intense," said Brookings scholar Vanda Felbab Brown on Twitter as news of Karzai's list spread. "He appears impervious even to input from top Afghans."

For all the public sniping, the US values Afghanistan's strategic location, and the government in Kabul would likely fall without American cash, so the two sides were expected to formally extend their unhappy marriage of convenience by another pragmatic decade. The US in particular has focused their hopes on working more closely with a new leader, because Karzai must step down as president this year.

They spent months haggling over a deal to keep American troops in the country after Nato's combat mission ends this year. Officials in Washington expected to be mapping out the details of a long-term military presence in Afghanistan by now, but instead they are making contingency plans for leaving the country entirely.

The bilateral strategic agreement is in limbo since Karzai surprised even some of his closest advisers by demanding new concessions after a draft was finalised late last year. Washington says that text is a final offer, and has warned that if it is not signed by Karzai or his successor they would be willing to pull out all forces, as they did in Iraq.

Since Karzai first backed away from the deal, a string of irritants between the two sides have made any kind of rapprochement look more and more distant.

They have sparred repeatedly and in public about the release of 88 men who the US considers dangerous militants, but the Afghan government says are mostly innocent victims, jailed without charges by an unjust foreign army.

Karzai's wider fears about US meddling were confirmed by revelations from the memoir of former defence secretary Robert Gates that US diplomats had tried to oust the president by manipulating a 2009 election. Washington had long denied any aim beyond cutting fraud and violence, but Gates described the effort as a "clumsy and failed putsch".

The most recent strain to ties has been reaction to the restaurant bombing and disputed accounts of a US airstrike several days before, both of which killed civilians.

Karzai delayed nearly a full day in condemning the attack on Taverna restaurant just a few hundred metres from his palace, and when he did, his statement took aim at the US airstrike as well. Among the dead were senior UN officials, and many diplomats bristled at a statement they felt drew tacit analogies between Nato forces supporting Afghan commandos a Taliban.

US forces admit two civilians were killed by the bombing; the government says the death toll was over a dozen, although officials also conceded the area was too dangerous for a government team to visit.

Then a dossier produced by presidential investigators researching the airstrike was revealed to include at least one image more than four years old, taken after an earlier bombing in another province. It cast doubt over all the other findings, and concerns were exacerbated by a government-organised press conference with villagers from the area. They claimed to personally know people in the recycled photograph, taken hundreds of miles away.

The next day, an Afghan television channel interviewed other villagers it said were relatives of the dead. They claimed the government "witnesses" were actually insurgent-linked fighters who had battled US and Afghan soldiers on the day of the airstrike.

The antagonism worries Afghan supporters of a US deal, who fear Karzai is throwing away the country's best chance of stability. They include most of the candidates to succeed him as president.

But Karzai's spokesman insists the president is committed to the deal. Long-term observers who have watched him shrug off past accusations of madness and outmanoeuvre US diplomats on issues including control of Bagram prison and night raids on Afghan homes, agree.

"He is playing things very well in these circumstances, he is not crazy at all," said one Kabul diplomat.

"He will sign it probably after the election, and with some conditions. He knows his future depends on Afghan power brokers and not the Americans … he is playing domestic policy, not international, now."