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

But outgoing President Hamid Karzai welcomed the timetable for the US's complete withdrawal and called on the insurgents to seize a "historic" opportunity to seek peace after more than a decade of war.

Outlining the US strategy to end America's longest war, President Barack Obama confirmed on Tuesday that the 32,000-strong US deployment in Afghanistan would be scaled back to around 9,800 by the start of 2015.

Those forces would be halved by the end of 2015 before 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 Americans 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 near the border with Iran.

Taliban insurgents responded to Obama's announcement by ruling out an end to fighting -- a grim indicator that Afghanistan's long, bloody war is far from over.

"Afghanistan Islamic Emirate condemns it and considers it a violation of sovereignty, religion and human rights," said a Taliban statement.

"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 (holy war) will continue against them."

But Karzai greeted the US withdrawal schedule by urging the insurgents to "use this historic opportunity... so that the war, which was launched (by the Taliban) on the pretext of the presence of foreign soldiers, ends".

"Finishing the US military presence, and assumption of security affairs by the Afghan forces, has been a main demand of the president, the government and the people of Afghanistan," he said.

- 'Afghanistan is not ready' -

Some observers in Afghanistan fear the withdrawal of US-led international troops will send the nation spiralling into chaos.

Fawzia Koofi, a female member of Afghanistan's parliament, 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, that 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."

The US drawdown relies on Afghanistan signing a long-delayed Bilateral Security Agreement laying out the terms and conditions of the US military presence in the country after this year.

Karzai has refused to sign the agreement, but both candidates vying to be his successor in next month's presidential election run-off -- Ashraf Ghani and Abdullah Abdullah -- have said they will sign the deal.

"I'm hopeful we can get this done," Obama said, as he outlined the end of US involvement in a conflict which began when American-led forces invaded Afghanistan to oust the Taliban and hunt Al-Qaeda leader Osama bin Laden after the 2001 attacks on New York and Washington.

"We're finishing the job we started," he said.

US combat operations will draw to a close at the end of 2014, meaning US troops would no longer patrol Afghan cities, towns or valleys from next year, Obama said.

US troops remaining in Afghanistan would be available to train Afghan forces while supporting counter-terrorism operations against Al-Qaeda remnants, Obama said, adding the withdrawal would be a "new chapter in American foreign policy".

Obama made an unannounced visit to US forces in Afghanistan on Sunday, spending four hours at Bagram, one of the secure bases that will house support troops after their combat mission ends.

There are currently about 50,000 NATO coalition troops in Afghanistan, with Britain contributing 5,200 and Germany 2,700.