SAMARRA, Iraq, Dec 4 (Reuters) - U.S. troops have sealed off an Iraqi town after militants forced its mayor and entire police force to resign, and residents said the Americans were preventing aid convoys from entering and people from leaving.

"The town of Siniya has been besieged since Thursday by U.S. forces, who are preventing anyone from entering or going out," said Hussein Mahjoub, the town's mayor until he and his fellow councillors resigned last week in fear of their lives.

The U.S. military said in a statement on Monday in response to questions from Reuters that checkpoints had been set up to "control movement and safeguard the citizens of the town" and that essential foodstuffs were being allowed through.

Police in the neighbouring oil refining city of Baiji, 15 km (10 miles) east, said the entire council and police force quit on Wednesday after receiving leaflets threatening them with death unless they resigned.

A day later, militants had destroyed the police station, cut off communications and were now in effective control of the town, they said. Reuters could not contact anyone in Siniya, 80 km north of Samarra.

"The police force and the city council are not working and people are complaining of a lack of food and fuel, which U.S. forces are preventing from entering the city," police Colonel, Faiq Huwaish told Reuters by telephone from Baiji.

Television footage filmed on Monday showed a line of trucks containing fruit, vegetables and other foodstuffs on a road outside the mainly Sunni town of 50,000, as a U.S. helicopter circled overhead.

AID CONVOY

Habib al-Milih, spokesman of the Iraqi Red Crescent Society in Salahaddin province said U.S. troops had been prevented their aid convoy from entering the town on Saturday.

Several trucks had banners saying where the aid had come from. "From the people of Tikrit to our brothers in Siniya," said a banner on one truck loaded with crates of potatoes.

Drivers complained they had been waiting since Thursday to enter the town, which is surrounded by high sand embankments, or berms, built earlier this year by U.S. troops to prevent infiltration by insurgents.

"We are still awaiting permission to go in. We have had long negotiations with the Americans, but it is clear their intention is to humiliate the people of Siniya," said Bassem Jayshi, a member of the Sunni Muslim Clerics Association.

This was contradicted by a U.S. statement which said U.S. and Iraqi forces were allowing the delivery of food, water and medical care "while asking for ... patience and cooperation".

Several hours after Reuters contacted the U.S. military to ask about conditions at Siniya, drivers said the blockade was briefly lifted to allow five trucks to enter the town.

"The checkpoints were established Saturday after insurgents executed several attacks on Iraqi Security Forces, Coalition Forces and Iraqi civilians in Siniya and the surrounding areas," the military said in response to questions.

Iraqi police said militants had attacked a U.S. patrol in the town with a roadside bomb on Wednesday, the same day the death threat leaflets were issued.

Mayor Mahjoub called on the Iraqi government to lift the siege since "it affected mostly the sick, women and children. There is a lack of food and fuel and it is preventing students from going to their colleges". (Additional reporting by Aseel Kami)