The move comes after it was disclosed that August was the worst month on record for US fatalities in Afghanistan.
The death of a US soldier in the east of the country last week pushed the death toll for the month to 45, marking August as the deadliest month for US forces in Afghanistan since the 2001 invasion toppled the Taliban government.
The figure was one more than the previous record set in July.
The rise in casualties since Barack Obama, the US president, ordered 21,000 extra troops to Afghanistan is likely to lead further questioning of the war in the US and other among other nations which have contributed to the Nato-led force in the country.
The icasualties.org website, which compiles and publishes statistics of the dead in the conflicts in Afghanistan and Iraq, says that 302 foreign troops have been killed in Afghanistan so far this year, up from 294 in the whole of 2008.
Bays said: "We have only reached August, there are still four months to go and yet we've surpassed the previous deadliest year - last year."
|At least 302 foreign troops have been killed
in Afghanistan in 2009 [Gallo/Getty]
Bays said McChrystal would be handing his assessment to Robert Gates, the US secretary of defence, and Anders Fogh Rasmussen, the Nato secretary-general.
He said: "He was going to announce this assessment on August 12 before the election, but there was concern that it could become a political issue and also that the security situation could change because of the polls.
"They are also changing the structure of the command here on the ground - McChrystal will deal with the overall strategy, but another general, David Rodriguez will deal with day-to-day war fighting.
"The other element of the plan that we hear is that Nato will get more involved in the training of the Afghan army and police.
"They are hoping that Nato nations that won't send more troops to fight might send more troops to train."
David Kilcullen, a senior adviser to McChrystal, told journalists in Australia on Sunday that hard fighting in Afghanistan was likely to last another two years and after that the Taliban would hopefully believe it was better to negotiate than continue combat.
That would be followed by a three-year transition to effective Afghan government and five-year overwatch period involving international forces as back-up, he said.
Britain, Canada and the Netherlands are the biggest contributors to the Nato-led effort in Afghanistan, after the United States.
A surge in British casualties as forces attempted to secure areas of the south before the elections, in which turnout was reportedly low, has already sparked debate there about the country's role in the conflict.