WASHINGTON, June 12 (Reuters) - The United States condemned as "unconscionable" on Thursday an incident in which Zimbabwe authorities seized a truckload of American food aid intended for hungry children and gave it out to government supporters instead.

U.S. officials said Zimbabwe military and police officials had hijacked a truck with 20 metric tons of food aid, including grains, beans and oil, and distributed it at a political rally of supporters of President Robert Mugabe's party.

"This unconscionable behavior must stop," Henrietta Fore, the administrator of the U.S. Agency for International Development, which had contracted the hijacked truck to distribute food, said in a statement.

"It is unacceptable for the government of Zimbabwe to steal food from hungry children," Fore said.

The theft happened last week, U.S. officials said, before Mugabe's government suspended the work of all international aid agencies in the southern African country.

Last week U.S. Ambassador James McGee accused Mugabe's government of using food as a weapon to get votes in Zimbabwe's run-off election on June 27 by only distributing aid to members of Mugabe's ruling ZANU-PF party.

State Department spokesman Gonzalo Gallegos said the truck hijacking was another example of using food as a weapon.

"Obviously, we're very disturbed to hear about this ... We believe that this must end," he told reporters.

"We call on the government, the Zimbabwean authorities, to immediately reinstate permission for all aid agencies to resume their life-saving assistance," Gallegos said.

"Failure to do so constitutes the government of Zimbabwean complicity in the assault, suffering and deaths of innocent citizens," he said.

About 4 million Zimbabweans rely on food aid in a country once seen as the region's breadbasket but where inflation is now a staggering 165,000 percent and unemployment 80 percent.

The USAID statement said the truck had made a scheduled stop at a school to deliver the food when a group of military officers, war veterans and police officers arrived and threatened the driver, forcing him to go to a nearby police station for protection.

The group harassing the truck had been dispatched by the governor of Manicaland, and was led by a Zimbabwe army colonel, the statement said.

But when the truck arrived at the police station, the governor told the war veterans to distribute the food to supporters of Mugabe's ruling ZANU-PF party at a rally nearby, USAID said.

Gallegos also condemned Zimbabwe's re-arrest of opposition leader Morgan Tsvangirai, president of the Movement for Democratic Change party, as well as the arrest of the party's secretary-general Tendai Biti.

"This tactic of repeated detention is yet another despicable example of the Mugabe regime's concerted effort to ensure that the opposition in Zimbabwe cannot mount an effective run off campaign," Gallegos said.

"If this government does not allow a free and fair run-off, they will have to pay in some way, shape or form in the end, and they will be held accountable," Gallegos said.

(Writing by Susan Cornwell, Editing by David Wiessler)

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