* Six million still need relief
* Anger rising
* World Bank may redirect $900 million in aid
* 500 stranded fuel tanker drivers
(Adds Zardari remarks, politicians pledging cooperation)
By Robert Birsel
SUKKUR, Pakistan Aug 14 (Reuters) - United Nations aid agencies have provided assistance to hundreds of thousands of victims of Pakistan's worst floods in decades but relief operations have yet to reach an estimated six million people.
The lives of 20 million people -- nearly 12 percent of the population -- have been disrupted by one of the worst catastrophes in Pakistan's history. Six million still need food, shelter and water, the UN said in a statement.
Highlighting the scale of the disaster, Prime Minister Raza Yusuf Gilani said in an Independence Day speech the country faces challenges similar to those during the 1947 partition of the subcontinent.
Thousands of families were torn apart after the bloody partition into Hindu-dominated India and Muslim-majority Pakistan that led to the flight of at least 10 million refugees in the greatest migration in recorded human history.
The floods, triggered by torrential monsoon downpours just over two weeks ago, engulfed Pakistan's Indus river basin, killing up to 1,600 people.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ For a graphic on Pakistan's floods, click
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Pakistan's government, overwhelmed by the disaster, has been accused of being too slow to respond to the crisis with victims relying mostly on the military and foreign aid agencies for help.
Anger is spreading, raising the possibility of social unrest. In Sindh province, flood victims complain of looting and there are signs of increasing lawlessness.
Gilani and opposition leader Nawaz Sharif, Pakistan's most popular politician, vowed to work together to tackle the crisis.
"Politics at this time is haram (forbidden by Islam)," Sharif said in a joint news conference.
Millions of Pakistanis, frustrated by political struggles at the best of times, want to know when their government will help.
"The government has given us half a carpet. We have received rice and medicine from the government but no tent," said 22-year-old labourer Zarsheed.
Pakistani President Asif Ali Zardari drew heavy criticism for going abroad to meet the leaders of Britain and France as the crisis unfolded, and not cutting short his trip.
"Despondency is forbidden in our religion. We consider it as a test from Allah for us. This is a test for us and for you," he told flood victims at a relief camp on Saturday. "We will try to meet all your wishes. We will build a new house for you. We will build a new Pakistan."
Despite the government's perceived failure in the crisis, analysts say a military coup is unlikely. The army's priority is fighting Taliban insurgents, and seizing power during a disaster would make no sense, they say.
It already sets security policies and influences foreign policy, and is described by some as a state within a state.
U.N. Secretary-General Ban Ki-moon is expected to meet Pakistani leaders in Islamabad on Sunday to discuss the flood crisis.
The economic costs of the flooding are staggering, making it tougher for the government to carry out strategic spending in former Taliban bastions to win public support.
The International Monetary Fund has warned of major economic harm and the Finance Ministry said it would miss this year's 4.5 percent gross domestic product growth target.
Wheat, cotton and sugar crops have all suffered damage in a country where agriculture is a mainstay of the economy.
Floods have affected about one-third of Pakistan, an area the size of a European country, says the U.N. Clean drinking water is needed for an initial target of six million people.
The floods roared down from the northwest to Punjab province to Sindh, where more flooding is expected. Sindh is home to Pakistan's biggest city and commercial hub Karachi. Floods have damaged mostly rural areas there, although concerns are rising that other urban centres are at risk.
Pakistanis are still at the mercy of the elements.
Scattered thundershowers with few heavy falls are expected in the upper northwest, upper Punjab, parts of the north and Kashmir over the next 24 hours, said the Meteorological Department.
Scores of villages have been wiped away. Some people only have a patch of land to stand on.
In the town of Muzaffagarh in Punjab, 500 fuel trucks line both sides of a highway. Like about two million Pakistanis, the drivers live in the open, sleeping on mats under their vehicles.
Along the same stretch of road, about 5,000 people live in and around the median along a 10 km (6 mile) stretch. Relief groups stop and hand out lentils and bread. Motorists throw water bottles out car windows at children who run alongside vehicles. (Additional reporting by Aija Braslina in SIGULDA, Tim Wimborne in NOWSHERA, Adrees Latif in MUZAFFAGARH, Robert Birsel in KHANPUR, Zeeshan Haider and Augustine Anthony in ISLAMABAD and Junaid Khan in MINGORA; Writing by Michael Georgy; Editing by Sanjeev Miglani)