Afghan authorities are trying to identify the suicide bomber who killed at least 12 people and injured 36 others outside an army base in Kabul.
Security is said to be tight in Kabul after yesterday's blast
The Taleban named a man it said was the bomber, but the Defence Ministry said the group's claim to have carried out Wednesday's attack was a "lie".
The UN has restricted the movement of staff, already under a night curfew.
The bombing was the deadliest since June when an attacker blew himself up killing 20 in a Kandahar mosque.
Eyewitnesses at the site said a man, dressed in military uniform, crashed his motorcycle into buses in a busy bus station on Wednesday afternoon.
The Nato-led International Security Assistance Force (Isaf) was helping investigation the blast, Defence Ministry spokesman Mohammad Zahir Azimi said.
Abdul Latif Hakimi, who claims to speak for the Taleban, told the BBC's Rahimullah Yusufzai in Peshawar that the suicide bomber was a Taleban loyalist.
He identified the attacker as Mullah Sardar Mohammad from Kabul.
Asked why Afghans were killed, Mr Hakimi said the attacker must have tried to target foreigners who serve as trainers at the Kabul military training centre.
Defence Minister Gen Rahim Wardak dismissed the Taleban's claim but blamed the blast on the "enemies of peace and stability" in Afghanistan.
"The Taliban tell lies. Let's wait for the investigation to be completed," he told the Associated Press news agency.
Experts were trying to identify the remains of the attacker, Gen Wardak said.
The Afghanistan non-governmental organisation, Safety Office, warned that it believed there could be another similar attack.
"We had indications about two guys riding motorcycles with explosives so we still assume there's another guy carrying explosives around," Christian Willach, its co-ordinator, told AFP news agency.
"We advised NGOs to restrict their movements over the last couple of days," he said.
The UN has also urged its staff to restrict movement in Kabul and avoid unnecessary travel.
"At the moment there is restricted movement while we assess the situation," UN spokesman Adrian Edwards said.