A series of attacks on American bases in Afghanistan, including a secret spy base, have raised fears that the Taliban will step up assaults on US forces ahead of the May 1 deadline to pull out.
Rockets twice struck a base used by military personnel working for the CIA in eastern Afghanistan last month, CNN reported, in apparent breach of the US-Taliban withdrawal deal signed last year.
Kandahar airfield which is used by US and coalition troops supporting the Afghan forces was then hit earlier this week.
The attacks came as Joe Biden is reviewing whether to stick with the Doha deal signed by Donald Trump, under which all US troops should leave the country by the end of this month. The Taliban have stopped attacking US forces since the Doha deal and are believed to have privately agreed not to attack the departing troops.
Mr Biden has said pulling out with only weeks to go will be tough and American officials have asked the Taliban to accept an extension as they try to revive talks to get a political settlement. The president has also suggested a US counter-terrorism force could remain in the country.
The Taliban have publicly said America must stick to the Doha deal and have threatened an escalation in their attacks, if they do not leave. International missions in Kabul have begun drawing down staff in preparation for an expected wave of violence.
Umer Karim, of the Royal United Services Institute think tank, said the attacks were likely to be a signal that the Taliban could mount a spring offensive and that all bases, including CIA counter terrorism bases, were unacceptable.
He said: “This of course sends a signal from Taliban that they will be launching a summer offensive and attacking American facilities.
“But also that they have the intel on every facility where there are American assets.”
Last month's rocket attacks hit Forward Operating Base Chapman, a classified US base in Khost province that played a major role in CIA operations hunting al-Qaeda in the border regions between Afghanistan and Pakistan.. The base was the scene of a 2009 suicide bombing when a Jordanian double agent killed seven Americans working for the CIA.