US commandos swooped into lawless Somalia Wednesday and rescued two Western aid workers held for three months by armed pirates in a daring pre-dawn raid approved by President Barack Obama.

In a rare US incursion into Somalia, elite Navy SEALs flying at least six military helicopters flew in to pluck an American, Jessica Buchanan, and Dane Poul Thisted to safety in a firefight that left all nine kidnappers dead.

Obama, who had been updated regularly about the kidnapping by his top defense staff, gave the green light late on Monday night for the risky operation on learning that 32-year-old Buchanan's health was in danger.

"Jessica's health was failing," Vice President Joe Biden said on ABC's Good Morning America show. "They concluded they should go at this time. The president gave the go."

Buchanan and Thisted, 60, employed by the Danish Refugee Council Demining Group helping to de-mine war-torn Somalia, had been held since October 25 when they were seized by gunmen in the semi-autonomous Galmudug region.

The two freed hostages were unharmed and flown to safety. Reports said they were being treated in Djibouti, where the US has its only base in Africa.

Obama was told by his top counterterrorism chief John Brennan of the rescue just over two hours before his State of the Union address on Tuesday.

The operation showed "the United States will not tolerate the abduction of our people, and will spare no effort to secure the safety of our citizens," the president said in a statement.

"This is yet another message to the world that the United States of America will stand strongly against any threats to our people."

Pentagon officials said a convergence of factors led to Monday's decision to launch the operation, but also refused to confirm many specific details, including what Buchanan is ailing from.

"There was a window of opportunity for mission success and we had information that suggested that one of the hostages, Ms Buchanan, might have a very serious medical condition that could be life-threatening," said Pentagon spokesman George Little.

Navy Captain John Kirby, another Pentagon spokesman, said the military had reason to believe Buchanan's condition was pre-existing when she was taken hostage "and also we had reason to believe that it was getting worse."

The commando raid on Somali soil is one of the first officially acknowledged by the United States, which admits to flying surveillance drones but rarely comments on reported missile strikes against Al-Qaeda-linked militants there.

The Horn of Africa nation has been without an effective government since it was plunged into a civil war two decades ago, allowing militias to flourish, with Islamist insurgents and pirate gangs ruling mini-fiefdoms.

Memories are also still raw in the United States of shocking scenes in 1993 when two US Black Hawk choppers were shot down and the bodies of 18 US soldiers dragged through Mogadishu's streets.

The first hint of the mission was when Obama was seen greeting Defense Secretary Leon Panetta on his way into Congress late Tuesday to give his State of the Union address, telling him: "Good job tonight. Good job tonight."

After the speech, Obama, still in the Capitol building with his wife Michelle at his side, called John Buchanan to tell him his daughter was safe. The White House released a photo of Obama during the phone call.

General Carter Ham, head of US Africa Command praised the "courageous, competent and committed" men who carried out the raid, at least 100 kilometers (60 miles) inside Somalia.

State Department spokeswoman Victoria Nuland said the US had been in "very close contact throughout with the government of Denmark."

The Pentagon would not confirm reports that the same unit of Navy SEALs who killed Al-Qaeda leader Osama bin Laden in a raid on his Pakistan compound last May was involved again.

Burnishing his credentials as the nation's commander-in-chief, Obama on Tuesday hailed bin Laden's death and told lawmakers that one of his proudest possessions was the SEAL flag carried on the raid.

Rights group Ecoterra International reported Wednesday that an American journalist and writer, Michael Scott Moore, had been kidnapped at the weekend in central Somalia and was being held by pirates alongside two hostages from Israel and the Seychelles taken from a hijacked Seychelles-flagged motorboat.

"We are aware of news reports that a US citizen has been kidnapped in northern Somalia and we are concerned about the individual's safety and well-being," the US State Department said.