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mericans on Thursday marked the 13th anniversary of the September 11, 2001 attacks, with somber ceremonies of remembrance in New York and Washington.

President Barack Obama, his wife Michelle and Vice President Joe Biden observed a moment of silence on the White House south lawn, along with about 300 staffers. Flags in Washington flew at half-mast.

Obama was then to attend a ceremony at the Pentagon '€” scene of one of the strikes.

In New York, relatives of those killed when hijacked airliners slammed into the World Trade Center gathered at Ground Zero to remember the dead.

The ceremony began, as usual, with a moment of silence at 8:46 am, when the first plane smashed into the Twin Towers.

Family members then began the long process of reading the names of the nearly 3,000 people who were killed in the attacks on New York, the Pentagon and in Shanksville, Pennsylvania.

This is the first year that the 9/11 museum in New York will be open on the anniversary of the attacks.

The anniversary comes just hours after Obama laid out his strategy to counter Islamic State (IS, also known as ISIL) fighters in Iraq and Syria '€” a new front in America'€™s unresolved confrontation with radical Islam begun in earnest 13 years ago.

Obama told Americans on Wednesday he had authorized US air strikes for the first time in Syria and more attacks in Iraq in a broad escalation of a campaign against the IS militant group.

Obama'€™s decision to launch attacks inside Syria, which is embroiled in a three-year civil war, marked a turnabout for the president, who shied away a year ago from air strikes to punish Syrian President Bashar al-Assad for using chemical weapons against his own people.

In a widely anticipated, 13-minute White House speech, Obama said he would hunt down Islamic State militants '€œwherever they are'€ in a drive to degrade and ultimately destroy the group, which has seized broad stretches of Iraq and Syria.

'€œThat means I will not hesitate to take action against ISIL in Syria, as well as Iraq. This is a core principle of my presidency: if you threaten America, you will find no safe haven,'€ he said.

Obama asked Congress to authorize US$500 million to train and arm '€œmoderate'€ Syrian rebels. The training would take place in Saudi Arabia.

It is unclear whether more American weapons and training can shift the battlefield balance toward the US-backed rebels, who are badly outgunned by IS, other militant groups and Assad'€™s forces.

Obama plans to expand the list of targets inside Iraq beyond several isolated areas. The US military has launched more than 150 air strikes in Iraq in the past month to help halt IS advances.

The new target list will include Islamic State'€™s '€œleadership, logistical and operational capability'€, as well as an attempt to '€œdeny it sanctuary
and resources to plan, prepare and execute attacks'€, the White House said.

US officials have warned it will take years to destroy IS, and Obama told Americans: '€œIt will take time to eradicate a cancer like ISIL,'€ the White House'€™s acronym for the militant group.

Obama will send 475 more American advisers to help Iraqi forces, which will bring to 1,600 the number there. Obama, determined to avoid a repeat of the Iraq war, stressed they would not engage in combat.

Responding to Obama speech, Britain'€™s foreign secretary says his country would not participate in air strikes on Syria, following an announcement from Washington that it would begin hitting targets inside the country.

Speaking Thursday after talks with his German counterpart Frank-Walter Steinmeier, Philip Hammond said Britain would not be '€œrevisiting'€ the issue after Parliament decided last year against participating in air strikes.

Germany often shuns taking part in combat operations and Steinmeier said his country also would not join in any air strikes.