Washington: US Ambassador Ryan Crocker to Kabul said the ability of Afghan security forces to respond to a wave of coordinated attacks today across Afghanistan were a "clear sign of progress."

"We've seen a very professional performance by Afghan security forces," Crocker told CNN today after militants launched a series of gun and suicide attacks which they said marked the start of a spring offensive.

"They are able to deal with events like this on their own. A clear sign of progress," he said on the programme "State of the Union."

The attacks however also "demonstrate why we need to be here," said Crocker, after the US embassy was put on lockdown during attacks that highlighted a precarious security situation as NATO prepares to withdraw combat troops by the end of 2014.

"To get out before the Afghans have a full grip on security, which is a couple of years out, would be to invite the Taliban and Al-Qaeda back in, and set the stage for another 9/11... that, I think, is an unacceptable risk," Crocker said, referring to 2001 terrorist attacks on New York and Washington.

A spokesman for NATO's International Security Assistance Force (ISAF) told AFP today Afghan forces, whose ability to withstand the Taliban after 2014 has been questioned, was taking the lead in countering the assaults on Kabul.

A residual US force is expected to remain past 2014 after the 130,000-strong US-led NATO troops end combat operations.

US commanders have meanwhile recently signalled eventual troop strength of Afghan forces would be dramatically scaled back from initial plans, due to concern about the cost that will be absorbed mostly by the United States and its allies.