Tanks to join Trump's July fourth parade

US Democrats are increasingly concerned about the cost and tone of pomp and ceremony at this year's Independence Day celebrations in Washington.

Battletanks are being spruced up to add military weight to July 4 celebrations in Washington. (AAP)

Battle tanks are getting some spit and polish to take part in July the Fourth celebrations in Washington, with Democrats accusing President Donald Trump of turning the event into a re-election campaign rally.

M1 Abrams tanks and other armoured vehicles were seen being transported on flat rail cars on Tuesday morning, a sign of the military firepower that will be featured, differing significantly from past annual celebrations of the 1776 Declaration of Independence

Amid questions about the cost and tone of the event, White House officials said the Republican president will avoid politics and stick to patriotic themes in his speech in front of the Lincoln Memorial in Washington on Thursday.

For decades, U.S. presidents have kept a low profile during the event, which typically draws hundreds of thousands of people to Washington's monument-lined National Mall for a concert and fireworks.

This year, Trump plans to speak at a military "Salute to America" that will feature military bands and flyovers from the U.S. Navy's Blue Angels and Air Force One, the modified Boeing 747 that transports U.S. presidents.

The event could also feature a B-2 bomber, F-35 and F-22 fighter jets, the Pentagon said. Air traffic at nearby Ronald Reagan National Airport will be suspended during the flyovers and the fireworks, according to the Federal Aviation Administration.

The administration has not said how much Trump's pumped-up July Fourth celebration will cost. The Pentagon postponed a military parade planned for last November after it estimated it could cost $90 million.

"It is unacceptable that the Interior Department is failing to inform Congress about how it plans to spend taxpayer money to fund the president's lavish July Fourth plans, which reportedly include special access to the National Mall for the politically connected," Democratic Senator Tom Udall said in a statement.

The anti-war group Code Pink said it had secured a permit to bring a "Baby Trump" blimp, depicting the president in diapers, to a protest in the Mall during his speech. However, the permit does not allow the balloon to be filled with helium so it can float.