A suicide bomber struck a rally in the Pakistani city of Quetta on Friday, killing at least 22 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, U.S. Defence Secretary Robert Gates said in Afghanistan where he is visiting U.S. troops.

More than 100 people were wounded in the blast in Quetta.

Earlier, the al Qaeda-linked Taliban took responsibility for triple bombings at a Shi'ite Muslim procession in the city of Lahore this week, challenging the civilian government further.

Aside from its struggles against homegrown Taliban, Pakistan is under intense American pressure to tackle Afghan Taliban fighters who cross the border to attack U.S.-led NATO troops.

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.

Wednesday's blasts in the eastern city of Lahore in which 33 people were killed was the first major militant attack since flood waters tore through the country over the past month.

It's revenge for the killings of innocent Sunnis, a spokesman for Qari Hussain Mehsud, mentor of the Taliban's suicide bombers, told Reuters by telephone from an undisclosed location.

In another attack on Friday in the northwest, a suicide bomber killed one person outside a mosque of the Ahmadi sect, who consider themselves Muslims but whom Pakistan has declared non-Muslims.

Pro-Taliban Sunni militants frequently attack religious minorities in as part of an overall strategy to destabilise the government.

Attention has focussed on the Pakistani Taliban again after U.S. prosecutors charged its leader, Hakimullah Mehsud, in the plot that killed seven CIA employees at an American base in Afghanistan last December.

The renewed violence suggests the Taliban are trying to hit the government as it struggles to cope with the floods, which have made millions homeless, destroyed infrastructure and crops and hammered the economy.

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.

ECONOMIC CRISIS

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 faced with an economic catastrophe, with the floods causing damage the government has estimated at $43 billion (£27.9 billion), almost one 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 country's economy cope with the devastation of the floods

Talks in Washington with a delegation led by Pakistan's Finance Minister Abdul Hafeez Shaikh on the terms of an $11 billion IMF loan program left him satisfied with the country's commitment to reforms, the IMF chief said.

Under the 2008 IMF loan program, Islamabad pledged to implement tax and energy sector reforms and give full autonomy to the State Bank of Pakistan.

(Writing by Michael Georgy; Editing by Miral Fahmy)

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