WASHINGTON -- It's been described as the greatest burden any commander in chief must bear.

Just days into his young presidency, a U.S. service member has died in military action authorized by Donald Trump. It's the first known combat death of a member of the U.S. military since Trump took the oath of office on Jan. 20 and underscores the gravity of the decisions he now makes.

Three service members were also wounded Sunday during the firefight with militants from al-Qaida in the Arabian Peninsula's branch in Yemen. The raid left nearly 30 others dead, including an estimated 14 militants. A fourth U.S. service member was injured when a military aircraft assisting in the mission had a "hard landing" nearby, according to U.S. Central Command.

"Americans are saddened this morning with news that a life of a heroic service member has been taken in our fight against the evil of radical Islamic terrorism," Trump said in a statement.

"My deepest thoughts and humblest prayers are with the family of this fallen service member," he said.

The names of the casualties were not released.

Planning for the clandestine counterterrorism raid begun before President Barack Obama left office on Jan. 20, but Trump authorized the raid, according to a U.S. defence official, who was not authorized to discuss details beyond those announced by the Pentagon and so spoke on condition of anonymity.

The U.S. has been striking al-Qaida in Yemen from the air for more than 15 years, mostly using drones. Sunday's surprise pre-dawn raid could signal a new escalation against extremist groups in the Arab world's poorest but strategically located country.

The action provides an early window into how the new president will put his campaign rhetoric into action when it comes to foreign intervention.

Trump had promised an "America first" approach and an end to the "era of nation building" if he won the White House. Many interpreted his language as isolationist and expected Trump to be more cautious about where the U.S. intervened.

At the same time, Trump had broadcast a stronger posture on the world stage. He pledged to beef up the military and said he aimed to achieve "peace through strength."

Sunday's raid was not the first time that the United States had conducted a counterterrorism raid on the ground in Yemen, but it was not the usual approach of striking from the air, the defence official said.

The raid was planned as a clandestine operation and not intended to be made public, but the loss of a service member changed that, the official said, adding that no detainees were taken in the operation.

An al-Qaida official and an online news service linked to the terror group said the raid left about 30 people dead, including women and children. Among the children killed was Anwaar, the 8-year-old daughter of Anwar al-Awlaki, a radical Yemeni-American cleric killed in a U.S. airstrike in Yemen in 2011, according to the girl's grandfather.

Nasser al-Awlaki told The Associated Press that Anwaar was visiting her mother when the raid took place. She was shot in the neck and bled for two hours before she died, he said.

In addition to killing the militants, White House Press Secretary Sean Spicer said U.S. forces "captured a whole host of information about future plots that's going to benefit this country and keep us safe."

The president "extends his condolences," he said on ABC's "'This Week." "But more importantly, he understands the fight that our servicemen and women conduct on a daily basis to keep this country safe."

Just over a week ago, suspected U.S. drone strikes killed three other alleged al-Qaida operatives in Bayda in what was the first-such killings reported in the country since Trump assumed the U.S. presidency.

Al-Qaida in the Arabian Peninsula, long seen by Washington as among the most dangerous branches of the global terror network, has exploited the chaos of Yemen's civil war, seizing territory in the south and east.

The war began in 2014, when Shiite Houthi rebels and their allies swept down from the north and captured the capital, Sanaa. A Saudi-led military coalition has been helping government forces battle the rebels for nearly two years.

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Ahmed al-Haj reported from Sanaa, Yemen. Associated press writers Maggie Michael in Cairo and AP National Security Writer Robert Burns in Washington contributed to this report.