President Hamid Karzai has lost faith in the US strategy in Afghanistan and is increasingly looking to Pakistan to end the insurgency, according to those close to Afghanistan's former head of intelligence services.

Amrullah Saleh, who resigned last weekend, believes the president lost confidence some time ago in the ability of Nato forces to defeat the Taliban.

As head of the National Directorate of Security, Saleh was highly regarded in western circles. He has said little about why he quit, other than that the Taliban attack on last week's peace jirga or assembly in Kabul was for him the "tipping point"; the interior minister, Hanif Atmar, also quit, and their resignations were accepted by Karzai.

Privately Saleh has told aides he believes Karzai's approach is dangerously out of step with the strategy of his western backers. "There came a time when [Karzai] lost his confidence in the capability of the coalition or even his own government [to protect] this country," a key aide told the Guardian.

Saleh believes Karzai has long thought this, but his views were crystallised in the aftermath of last year's election when millions of votes were found to be fraudulent; Karzai blamed the US, UK and United Nations for the fraud.

According to the source, Saleh is deeply concerned by Karzai's noticeably softer attitude towards Pakistan. The president has long dropped his past habit of excoriating Pakistan for aiding the Taliban.

Saleh also echoes complaints of US commanders that Karzai refuses to behave like a commander-in-chief, and is not publicly leading the counterinsurgency campaign devised by Stanley McChrystal, the US commander of Nato forces.

In London today, the US defence secretary, Robert Gates, warned that progress needed to be made. "In all coalition countries the public expects to see us move in the right direction [but] will not tolerate the perception of a stalemate, where we are losing our young men."

Gates also warned of "a high level of violence, especially this summer", as US forces push deeper into southern provinces where the Taliban are strongest. Today in the south, insurgents shot down a Nato helicopter, killing four US troops, while a British soldier died in a separate attack. In Islamabad in Pakistan, an assault on a depot by insurgents destroyed 50 lorries belonging to the Nato military supply chain.

Saleh's resignation on Sunday, along with Atmar, was a huge blow to the government, which is otherwise largely lacking in high-calibre senior officials. He was strongly supported by the CIA and MI6, and had a reputation as hardworking and honest. His six years at the head of what is probably Afghanistan's least dysfunctional state body gave him extensive access to Karzai, a man he still regards as a "patriot" and whom he is reluctant to publicly criticise.

But, according to Saleh's aides, the final straw came last Sunday when Karzai apparently questioned his loyalty during a stormy meeting at the presidential palace, appearing not to believe Saleh and Atmar's account of how two insurgents armed with rocket launchers, one dressed as a woman, were able to get so close to a meeting of 1,600 national leaders.

Saleh's colleagues say that Karzai even accused the two men of a plot with the Americans and the British to wreck his peace plan.

Saleh's friends say that, because Karzai believes Nato is unable to deal with insurgent sanctuaries on the eastern border, he is looking for an alternative strategy: rather than use western support to "harden" Afghanistan against its neighbour, he is instead striking a less robust attitude to Pakistan and the Taliban.

The former intelligence head is outraged by the tone of the jirga, which stressed the role of the weak and corrupt Afghan state in fomenting insurgency. And he was appalled by Karzai's post-jirga announcement that a commission would be set up to release Taliban prisoners not held on solid evidence – such evidence in many cases came from Saleh's directorate.

But Karzai is not alone. McChrystal and diplomats have for months argued that Pakistan is rethinking its support for the Taliban after deadly attacks on Pakistani cities, and say arrests of Taliban commanders, such as Mullah Abdul Ghani Baradar in February, are proof of the new mood in Pakistan – something Saleh disputes.

Casualties rise

There has been an increase in violence in Afghanistan as US commanders put the final touches to a plan to secure the Taliban's southern heartland of Kandahar, an operation they hope will turn the tide of the war.

The latest casualties yesterday took Nato's toll to 29 deaths in nine days, according to an AP count. The United States, whose some 94,000 troops vastly outnumber the rest of the allies' contributions in Afghanistan, has lost 17 service members since Sunday.

The downing of an American Blackhawk helicopter in Helmand province saw four American crew die, but a couple of wounded British soldiers being carried on board survived and were transferred to another Blackhawk. Their injuries are said to be not life-threatening. Helmand provincial spokesman Daoud Ahmadi said the helicopter was shot down about midday in Sangin district during an operation involving Nato and Afghan security forces. Taliban spokesman Qari Yousef Ahmadi said the insurgents shot off two rockets to down the helicopter. AP