Taliban insurgents denounced US plans to keep troops in Afghanistan until the end of 2016, threatening on Wednesday to wage war against the "occupation" until the very last foreign soldier pulls out.

Outlining the US strategy to end its longest war, almost 13 years after the Sept 11 attacks, US President Barack Obama had confirmed on Tuesday that the 32,000-strong US deployment in Afghanistan would be scaled back to about 9,800 by the start of 2015.

Those forces would be halved by the end of 2015 before eventually being reduced to a normal embassy presence with a security assistance component by the end of 2016.

But underscoring the instability still roiling Afghanistan, two US citizens were slightly wounded in an attack on a US consulate vehicle in Afghanistan's western city of Herat on Wednesday.

An unidentified gunman on a motorcycle fired a rocket-propelled grenade at the US vehicle in Herat, five days after insurgents attacked an Indian diplomatic mission in the same city, which is near the Iranian border.

Taliban insurgents responded to Obama's announcement by ruling out an end to fighting until a complete withdrawal of US forces had taken place - a grim indicator that Afghanistan's long, bloody war is far from over.

"Now that Obama has announced that he will keep around 10,000 troops until the end of 2016 and continue their occupation, Afghanistan Islamic Emirate condemns it and considers it a violation of sovereignty, religion and human rights," a Taliban statement said.

The statement, which used the insurgents' name for the country, appeared to inflate the number of troops scheduled to remain under Obama's plan.

"The American leaders should do now what they plan to do two years later. Even if one American soldier is in Afghanistan, it is not acceptable to our nation and jihad will continue against them."

'Not ready'

The threats reaffirmed the fears raised by some observers in Afghanistan that a complete withdrawal of US-led international troops would send the nation spiraling into chaos.

Fawzia Koofi, a female member of Afghanistan's National Assembly, posted on Twitter: "We are proud of the moral/self-esteem of our security forces, however, I had expected more than ten thousand troops to continue post 2014".

Others warned that the lingering militant threat could lead Afghanistan down the path of Iraq, where violence is at its highest level since 2008.

"Afghanistan is not ready," said security analyst Mia Gul Wasiq. "If they withdraw irresponsibly, Afghanistan will become like Iraq.

"We have not been able to establish a strong government. ... The US has not done its job, which was to root out terrorism from Afghanistan. The war and terrorism is still there; their job is not done. So their plan and timetable on paper is not practical."