Security forces on Tuesday also searched houses for more suspected attackers.
Police commander General Mohammed Akbar said the bodies of the two attackers who killed themselves in the blasts appeared to be those of Arabs.
But a spokesman for Nato's peacekeeping force, Major Andrew Elmes, said the bodies were believed to be those of Afghans.
Akbar said police scouring the scene of the second car bombing found six more burned bodies in a ditch.
"Al-Qaida is definitely behind this attack," he said. "Only al-Qaida has the capability to do this."
Hours after the attacks late on Monday, a purported Taliban spokesman claimed responsibility.
If al-Qaida is confirmed to also have a hand in it, it would reinforce fears that the terror network is still working with the Taliban, which US-led forces overthrew from power in 2001 for harbouring Osama bin Laden.
Such seemingly coordinated assaults are unprecedented in Afghanistan and were the first major attack on foreign troops in Kabul in a year.
"Al-Qaida is definitely behind this attack"
Police commander General Mohammed Akbar
They came during the deadliest year of rebel violence since the toppling of the Taliban and have reinforced fears that insurgents are copying tactics used in Iraq.
An Afghan child was also killed in the second bombing.
In the initial blast, a German peacekeeper died. The new deaths brought the toll to eight, in addition to the death of the two attackers.
Peacekeepers fatally shot three other men as they raced in a car towards the scene of the blasts, fearing it was another suicide attack.
State of alert
UN spokesman Adrian Edwards said the world body had gone to a heightened state of alert in the city, and only essential staff were allowed to come to work.
Fears of more attacks have also prompted Nato's peacekeeping force to go on a higher state of alert, Elmes said.
Fears of more attacks prompted
Nato forces to go on state of alert
Roadblocks have been set up around the city, and police were searching houses for more attackers.
"We're on alert but it's very hard to prevent suicide attacks with hundreds of vehicles on the roads," said Akbar, the police general.
The blasts underscore the challenges facing US-backed President Hamid Karzai as he struggles to support his nation's fledgling democracy.
UN Secretary-General Kofi Annan expressed concern at the upsurge in violence in Afghanistan and strongly condemned Monday's bombings.
"The secretary-general calls upon the government of Afghanistan, the (US-led) coalition forces and the International Security Assistance Force to take all necessary measures to address the security situation," UN deputy spokeswoman Marie Okabe said.
Kabul, home to about 3000 foreigners and patrolled by thousands of Nato peacekeepers, had been regarded as one of the country's safest places, despite a flurry of kidnappings over the past year.