A suicide bomber struck at a rally in the Pakistani city of Quetta on Friday, killing at least 54 people in the second major attack this week, piling pressure on a government struggling with a flood crisis.
The attack on the Shi'ite rally expressing solidarity with the Palestinian people came as the United States said the devastating floods are likely to delay army offensives against Taliban insurgents.
"Unfortunately the flooding in Pakistan is probably going to delay any operations by the Pakistani army in North Waziristan for some period of time," US Defense Secretary Robert Gates said in Afghanistan where he is visiting US troops.
Senior police official Hamid Shakeel told Reuters that at least 54 people were killed and about 160 wounded.
Dozens of dead and wounded lay in pools of blood after the blast that also engulfed vehicles in flames.
Hours later, the al Qaeda-linked Taliban took responsibility for the attack, saying it was revenge for killings of radical Sunni clerics by Shi'ites, further challenging the civilian government struggling to cope a month after the floods.
"We take pride in taking responsibility for the Quetta attack," Qari Hussain Mehsud, the mentor of Taliban suicide bombers, told Reuters by telephone from an undisclosed location.
Earlier in the day, the Taliban claimed responsibility for bombings on Wednesday at a Shi'ite procession in the eastern city of Lahore in which at least 33 people died. These blasts were the first major attack since flood waters tore through the country. The Taliban and their allies often target religious minorities in a campaign to destabilise the government.
Aside from its battles against homegrown Taliban, Pakistan is under intense American pressure to tackle Afghan Taliban fighters who cross the border into Pakistan's lawless tribal areas to attack U.S.-led NATO troops.
The United States has stepped up missile strikes by pilotless drone aircraft against militant targets in Pakistan's Pashtun tribal lands since the start of 2010. On Friday, U.S. drones fired missiles at two targets in North Waziristan tribal region, killing seven militants, including two foreigners, intelligence officials said.
Pakistan has said the army would decide when to carry out a full-fledged assault in North Waziristan, where Washington says anti-American militants enjoy safe havens, at the time it considers appropriate.
In another attack in the northwest, a suicide bomber killed one person outside a mosque of the Ahmadi sect, who consider themselves Muslims but whom Pakistan declares non-Muslims.
Attention has focused on the Pakistani Taliban again after U.S. prosecutors this week charged its leader, Hakimullah Mehsud, in a plot that killed seven CIA employees at an American base in Afghanistan last December.
Islamist charities, some of them linked to militant groups, have at the same time joined in the relief effort for the millions affected by the worst floods in the nation's history.
U.S. officials are concerned that the involvement of hardline groups in flood relief will undermine the fight against militancy in Pakistan as well Afghanistan.
Anger is spreading over the government's sluggish response to the floods, raising the possibility of social unrest.
"The scope and scale of this discontent will depend on the government's performance in preventing a second wave of deaths, loss of livelihood, and shortages of essential food items," said Eurasia consultancy group.
Pakistan is also facing economic catastrophe, with the floods causing damage the government has estimated at $43 billion, almost a quarter of the south Asian nation's 2009 GDP.
Some relief has come from the International Monetary Fund (IMF). It will give Pakistan $450 million in emergency flood aid and disburse funds in September to help the economy cope with the devastation.
Talks in Washington with a delegation led by Pakistan's Finance Minister Abdul Hafeez Shaikh on the terms of an $11 billion IMF loan programme left him satisfied with the country's commitment to reforms, IMF chief Dominique Strauss-Kahn said.
Under the 2008 IMF loan programme, Islamabad promised to implement tax and energy sector reforms and give full autonomy to the State Bank of Pakistan.