BOSTON, Massachusetts (AFP) - The surviving Boston bombing suspect has not yet been interrogated as he recovers in hospital from wounds sustained in a shootout, police said on Sunday.

Dzhokhar Tsarnaev, 19, is in "critical but stable condition," Boston Police Department Commissioner Ed Davis told reporters.

He was taken into custody bloodied and seriously wounded Friday after an hours-long manhunt and an earlier shootout that killed his older brother Tamerlan, 26.

"The suspect is not yet able to be interrogated by police," Davis said.

Officials have invoked a "public safety" legal exception that will allow them to question Tsarnaev without reading him his so-called Miranda rights to remain silent and to consult a lawyer.

Yet "we don't know if we'll ever be able to question the individual," Boston Mayor Thomas Menino earlier told ABC television's This Week programme.

Reports said charges against Tsarnaev could come as early as Monday.

Authorities have yet to disclose the exact nature of Tsarnaev's injuries as he receives treatment at Beth Israel Deaconess Medical Center, where some of the 180 people wounded in the blasts are also being treated.

Three people died in the attacks, the worst to take place in the United States since the suicide airliner bombings on Sept 11, 2001.

The police chief said the FBI may reopen Copley Square, the area where the double explosions took place by the finish line of the Boston Marathon on Monday.

Speaking on CBS television's Face The Nation, Mr Davis said the Tsarnaev brothers were armed for another attack.

"They had IEDS," Mr Davis said referring to improvised explosives devices.

"They had homemade hand grenades that they were throwing at the officers.

"The scene was loaded with unexploded improvised explosive devices that actually we had to point out to the arriving officers and clear the area," he added, noting that one IED was found in a Mercedes sports utility vehicle the brothers had abandoned.

"This was as dangerous as it gets in urban policing." He said federal authorities were trying to track down how and where the two suspects obtained the firearms and explosive devices.

Some Republican lawmakers have called for declaring the teenager an "enemy combatant," which would give him the same status as Guantanamo "war on terror" detainees.

But Senator Lindsey Graham has since nuanced his comments, stressing that Tsarnaev should only be considered an enemy combatant for interrogation purposes, and should receive a civilian trial in federal courts, rather than in a military tribunal like foreign suspects with that designation.

Critics have insisted that because Tsarnaev is a naturalised US citizen and authorities have found no ties between him and terror groups so far, he should be granted a criminal civilian trial.

Agents applied the Miranda rights security exception in a terror case involving Nigerian man Umar Farouk Abdulmutallab, who tried to blow up an airliner headed for Detroit, Michigan on December 25, 2009.

He was not read his rights before FBI agents interviewed him at the hospital where he receiving strong painkillers during treatment for his injuries.

But a judge ruled that statements Abdulmutallab made during those interviews could be used in trial.

He later pleaded guilty and was got four consecutive life sentences for his botched plot linked to Al-Qaeda's branch in Yemen.