Afghan President Hamid Karzai has removed the head of the intelligence service, Rahmatullah Nabil.
Mr Karzai's office said a decision had been taken to limit the term of the office to two years.
However, there are reports that Mr Karzai was unhappy with the agency's performance in combating the militant Taliban movement.
Mr Karzai is also expected to announce replacements for two security ministers who were dismissed by parliament.
Interior minister Bismillah Khan and Defence Minister Abdul Rahim Wardak were removed by MPs for failing to tackle insecurity and over corruption allegations.
A statement from Mr Karzai's office thanked Mr Nabil for his service and said he would take up a post as ambassador to a foreign country.
Reports say that Mr Khan will be re-introduced to the cabinet as defence minister.
Senior officials told the BBC that the current deputy interior minister Mujtaba Patang will be promoted to replace Mr Khan as interior minister.
Both appointments have to be approved by parliament, which may cause friction between parliament and Mr Karzai.
"We hope that we get new faces rather faces which have already been examined and which the people are not happy with," Hajji Almaas Zahed, an MP from Parwan province, told Reuters.
The moves comes amid concerns over the ability of the Afghan armed forces to combat the Taliban, and to prevent them infiltrating the security forces.
Most of the 130,000 Nato-led troops foreign troops in the country will leave by the end of 2014, and are gradually handing over responsibility for security to their Afghan counterparts.
In 2010, Mr Nabil's predecessor as head of the National Directorate of Security (NDS), Amrullah Saleh, resigned from his post, saying he disagreed with Mr Karzai's policy of encouraging talks with the Taliban.