A large blast rocked Kabul on Thursday evening, followed by gunfire in the Afghan capital's central district that houses many embassies and foreign compounds, AFP journalists reported.
The blast came after a Taliban suicide bomber rammed his explosives-packed car into a British embassy vehicle in Kabul earlier in the day, killing one Briton and five Afghans in the latest attack to highlight fragile security as NATO troops withdraw.
Police were unable to give immediate details on the latest attack, with sporadic gunfire erupting 30 minutes after the initial blast.
Attacks across Kabul have increased in recent weeks as US-led NATO forces prepare to wrap up their 13-year combat mission against Taliban insurgents at the end of this year.
Fears are growing that Afghanistan could tip into a cycle of violence as the NATO military presence declines, with the national security forces already suffering high battlefield casualties.
At least eight blasts have hit Kabul over the last 10 days, including attacks on foreign compounds and on a female Afghan member of parliament who was injured in a suicide bombing that targeted her car.
Afghanistan suffered its deadliest attack of 2014 on Sunday when a suicide bomber struck at a volleyball match in the eastern province of Paktika, killing 57 people.
About 12,500 foreign troops are set to stay on into 2015 after the NATO combat mission ends, to train and support the Afghan army and police.
A British security contractor and an Afghan employee of the British embassy died in Thursday's earlier blast, which hit their vehicle on a stretch of road notorious for suicide attacks.
At least four Afghan bystanders were also killed, and more than 30 others injured.
British Foreign Secretary Philip Hammond condemned the attack on the embassy vehicle as "senseless and cowardly".
"It reminds us once again of the risks our personnel take every day to keep trying to help Afghans build a better future -- and by helping them do so, protect our own security and own interests," he told a press conference in Rome.
A second British contractor with security firm G4S was injured in the bombing, which threw the embassy's 4x4 vehicle onto its side.
The roof was blown off and car parts scattered across the Jalalabad road, a main route where many fortified compounds and military facilities are located.
President Ashraf Ghani, who came to power in September, has vowed to bring peace to Afghanistan after decades of conflict, saying he is open to talks with the Taliban, who ruled Kabul from 1996 to 2001.
Ghani finally emerged as president in September after signing a power-sharing deal with his poll rival Abdullah Abdullah.
Both men claimed to have won fraud-tainted elections in a prolonged stand-off that caused political paralysis in Kabul and fuelled worsening violence nationwide.