• Hearing suspended for the day as Petraeus recovers • Dehydration, jetlag and lack of food cited as causes

America's top commander, General David Petraeus, fainted during a congressional hearing as he was being grilled by senators sceptical about US strategy in Afghanistan.

His collapse came about an hour into the hearing as a Republican senator, John McCain, questioned him about recent setbacks. McCain stopped mid-sentence, his face frozen, as Petraeus slumped forward from his seat on to the witness table.

The hearing was suspended as Petraeus, looking dazed, was led out by army colleagues. He returned 20 minutes later, blaming not McCain's questions but dehydration.

He told the senators he wanted to continue but the chairman of the Senate armed services committee, Carl Levin, overruled him, and postponed the hearing until tomorrow morning.

The hearing is being held against a backdrop of growing unease about the war in Afghanistan. A long-trumpeted offensive against the Taliban in Kandahar province had to be delayed until at least September, while there are fresh doubts over the loyalty to the US of the Afghan president, Hamid Karzai.

Casualties have been climbing in the last few weeks, with five Nato fatalities today, three of them British. Two of the British soldiers were shot dead in Helmand province and a third died from wounds sustained in an exchange of fire on Sunday, also in Helmand.

In Congress, both McCain and his Democratic colleague, Levin, pressed Petraeus on whether it was feasible for US forces to begin withdrawing from Afghanistan in July next year, the deadline set by the US president, Barack Obama.

"I am deeply concerned about our campaign in Afghanistan. Many of the key trends seem to be heading in a bad direction, perhaps even signalling a mounting crisis," McCain said.

In the opening first hour, McCain and Levin both kept returning to the withdrawal deadline. Petraeus, 57, a four-star general who was treated last year for prostate cancer, seemed sprightly at the start of the hearing. He had returned to Washington to give evidence at the end of a week-long overseas trip and could have been suffering from jetlag or stress.

In the five minutes before he fainted, he turned pale, his eyes began to glaze over and his speech slowed down as if he had trouble gathering his thoughts. He was repeatedly sipping from a glass of water.

Reflecting opinion among politicians in Washington that there will be only a token withdrawal next year and that the US will be in Afghanistan for years to come, Levin asked Petraeus if he fully supported Obama's deadline. Petraeus, who had earlier in the hearing qualified the deadline, saying it would be based on conditions on the ground, hesitated for what seemed like an embarrassingly long time. He finally said the military had to be careful with timelines.

Levin asked him if that was a qualified yes, a qualified no or a non-answer. Petraeus said it was a qualified yes.

McCain questioned him about an Afghan intelligence chief liked by the US but sacked by Karzai last week who claimed Karzai was losing faith in the US and planned to shift his loyalty to the Taliban. It was about then that Petraeus collapsed. After being escorted out, Levin, McCain and other senators milled around their desks. Levin made a brief announcement to say Petraeus was recovering: "He's eating. He probably didn't have enough water to drink coming in here this morning."

Petraeus returned shortly afterwards to applause, shook hands with the senators and resumed his seat. "I was feeling a bit light-headed there," he said. "It was not senator McCain's questions. I just got dehydrated."

Temperatures in Washington in the last few days have reached over 32C (90F), added to which Petraeus said he had not had breakfast that morning. His spokesman, Colonel Erik Gunhus, added jet lag to the list of possible explanations.

Petraeus, who is credited in the US with having turned the war around in Iraq, is being touted by supporters as a potential Republican candidate for the 2012 presidential race. Petraeus has denied he is planning to stand.

Question marks over candidates' health have ruled out would-be candidates in the past.