The Pakistani Taliban on Tuesday claimed responsibility for a suicide bombing at UN World Food programme (WFP) offices in Islamabad.
The Monday afternoon attack, which killed five employees of the WFP, prompted immediate closure of all UN offices across the country "until further notice".
A Taliban spokesman told reporters by phone that his group was behind the bombing carried out by a suicide attacker who entered the WFP building disguised as a paramilitary soldier.
"We proudly accept the responsibility for the suicide bombing at UN office in Islamabad," said Azam Tariq.
According to Tariq, the attack was carried out to avenge the US drone attacks and bombing raids by Pakistan jets and helicopter gunships in South Waziristan, a lawless tribal district that borders Afghanistan.
He vowed Taliban would conduct more such attacks on Pakistani, UN and US targets inside Pakistan.
The blast came as Pakistan is reportedly preparing for a major assault in South Waziristan against the group of former Taliban chief Baitullah Mehsud, who died in a US missile attack in early August.
Interior Minister Rehman Malik told reporters in Islamabad Tuesday that the security forces had already started "a targeted operation" in the tribal district. He said if further action were needed, a full-fledged offensive would be launched at "an appropriate time".