* U.S. gives additional $20 million in aid
* Appeals for other nations to follow
* Concern grows over political and security fallout
By Sue Pleming
WASHINGTON, Aug 10 (Reuters) - The United States announced an additional $20 million to help Pakistani flood victims on Tuesday amid growing concern over the political, economic and security ramifications of the disaster.
The new aid brought to $55 million the amount of funds committed by Washington so far to flood relief efforts in Pakistan, along with U.S. military helicopters that have been airlifting survivors trapped by the worst floods in 80 years.
"It is a sign of our continuing commitment as this drama continues to unfold. It is very important to note that it is not just an immediate humanitarian crisis," said senior State Department official Dan Feldman in announcing the new funds.
"This is very much a medium-to-longer term issue with food security, with the economic infrastructure and with needs that will be ongoing for many months, if not years," said Feldman.
The floods have killed about 1,600 people and affected more than 14 million more, wiping out crops and livestock in areas that were among the most unstable in Pakistan to begin with.
Hundreds of roads and bridges have been destroyed and waters have not yet crested, meaning the situation could get worse in Pakistan, a U.S. ally in fighting militants in neighboring Afghanistan.
There is concern that Islamist charities with links to militant groups have been seeking to fill the gap left by what many see as an inadequate response by Pakistan's government, which has been overwhelmed by the scale of the disaster.
White House spokesman Bill Burton said he hoped militants were not using the tragedy to inflict even more pain on the population.
"The concern is that every single day you have got people who are intent on creating havoc in order to disrupt democracy and harm their way of life there," said Burton. "Right now we are worried about the floods and worried about the insurgency and we are dealing with those issues as appropriate."
Pakistan, whose air force chief was in Washington on Monday, has appealed for more helicopters as well as other transport and rescue aircraft to deal with the crisis.
Pentagon spokesman Geoff Morrell said the defense department was looking into adding to the six U.S. helicopters sent from Afghanistan but this could only be done with Pakistani approval and coordination.
"We are obviously best-positioned to assist in the disaster relief and we are looking very aggressively at what other assets we are able to contribute," said Morrell.
Pakistan expert Lisa Curtis urged the administration to "pull out all the stops" and deliver as much assistance as possible if it wanted to curb the influence of militants.
"Extremist groups understand that the political fault lines are shifting and they know how to take advantage of the situation and they will remember the groups who were there in their time of need," said Curtis of the Heritage Foundation.
Adding to concerns, is frustration over how President Asif Ali Zardari has personally dealt with the floods, especially after he chose to continue a European trip despite the crisis.
U.S. officials are also scrambling to determine whether Washington's ambitious nonmilitary aid program -- amounting to $7.5 billion agreed by Congress over the next five years -- will have to be adjusted to deal with the latest emergency.
"There are some other now, very critical needs on the table," said Mark Ward, a senior U.S. aid agency official. (Editing by Chris Wilson)