United States can't act alone in the world: Obama

President Barack Obama insisted on Saturday that the United States cannot act alone in the world as he outlined a new national security strategy aimed at cementing his break with the Bush era's more unilateral approach.

Setting out his vision for keeping America safe as it fights wars in Afghanistan and Iraq, Obama put international cooperation at the centre of his foreign policy, in contrast to what critics derided as the "cowboy diplomacy" of his predecessor, George W Bush.

"The burdens of this century cannot fall on our soldiers alone, it [sic] also cannot fall on American shoulders alone," Obama told graduating cadets at the US Military Academy at West Point. "Our adversaries would like to see America sap its strength by overextending our power."

Obama's aides had cast his speech as a preview of his national security strategy -- a policy document required by law of every US president — before it is released next week.

Bush laid out in 2002 what is commonly known as the 'Bush Doctrine' asserting that the United States would launch pre-emptive war against countries as well as terrorist groups deemed a threat to the United States. What followed was the US-led invasion of Iraq.

The Obama administration has fuelled speculation that the president's new strategy will back away from that controversial concept and instead stress the need to prevent attacks through multilateral ties and sound intelligence.

Though Obama gave no clear signal on this issue in his West Point speech, he did assert that the only reason US forces continued fighting in Afghanistan was because "plotting persists to this day" there by al-Qaeda extremists behind the attacks of September 11, 2001, on the United States.

Obama said the United States must strengthen existing alliances, build new partnerships, and promote human rights worldwide.

"We are clear-eyed about the shortfalls of our international system," he said. "But America has not succeeded by stepping out of the currents of cooperation."

Obama also kept up his outreach to the Muslim world. While accusing al-Qaeda of distorting Islamic values, he avoided using terms like "war on terror" and "Islamo-fascists" that Bush employed regularly and which alienated many Muslims.

"Extremists want a war between America and Islam, but Muslims are a part of our national life," Obama said.

Obama has been widely credited with improving the tone of US foreign policy but is still struggling with serious challenges ranging from nuclear standoffs with Iran and North Korea to sluggish West Asia peace efforts.

Providing a status report on Afghanistan, Obama warned of tough fighting ahead to break the momentum of a resurgent Taliban. "There will be difficult days ahead," the president said. "But we will adapt, we will persist, and I have no doubt that together with our Afghan and international partners, we will succeed in Afghanistan."

When Obama last visited West Point in December, he used it for a televised speech unveiling his plan to send 30,000 more US troops to Afghanistan coupled with a promise to start bringing forces home in July 2011.

The buildup is nearing completion. It is considered vital to a US-led offensive expected in coming months in the Taliban stronghold of Kandahar that could weigh heavily on the success or failure of Obama's approach.

Obama spoke just over a week after hosting Afghanistan's president Hamid Karzai at the White House, where they presented a unified front in a bid to show their differences were behind them.

But that did not mean the Obama administration or US lawmakers, who hold the purse strings, had suddenly gained full confidence in Karzai as a credible partner.

Obama's challenge is to convince a sceptical American public and Congress that the war is worth fighting and funding and to keep Afghanistan from becoming a political liability in a congressional election year.