President Barack Obama on Tuesday praised France as a model ally in a world that must do more to meet common threats, laying on the full flourish of a state visit for President Francois Hollande.

Obama hailed France, an increasingly key partner in the fight against Islamic extremism in Africa, nuclear proliferation and climate change, as he welcomed Hollande on the White House lawn.

The presidents stood side-by-side framed by the red, white and blue of the Stars and Stripes and the French tricolor as a 21-gun salute split the air and ranks of troops in dress uniform stood to attention.

"Like generations before us, we now have the task not simply to preserve our enduring alliance but to make it anew for our time," Obama said.

"No one nation can meet today's challenges alone or seize its opportunities.

"More nations must step up and meet the responsibilities of leadership, and that is what the United States and France are doing together."

While disengaging from wars in Iraq and Afghanistan, Washington has pressed its European allies to do more to protect their own security in operations like the Libya war -- albeit with military and logistical support that the only the US superpower can provide.

Hollande paid tribute to the sacrifice of US soldiers in World War II and said the two old allies stand for bedrock principles of universal rights.

"Today France and the United States still stand side by side to make these values prevail," Hollande said.

"We stand together with the United States to address the threats of the proliferation of weapons of mass destruction and chemical weapons, together to solve the crises faced by the Middle East, together to support Africa's development, and together to fight global warming and climate change."

Earlier, a senior US official praised France for helping quell unrest and extremist violence in parts of Africa.

"The French role has been key to achieving success in Mali and the French role in trying to bring about security and peace in CAR are very, very important," the US top diplomat for Africa, Linda Thomas-Greenfield said.

Hollande will be without a date at the state dinner later Tuesday in a huge marquee sumptuously decorated with French touches, after turmoil in his love life made global headlines.

Before then, Obama and Hollande will hold a press conference and the French president will have lunch with America's French speaking Secretary of State John Kerry.

The authenticity of the Franco-US alliance, often tumultuous, lies in its resilience: ties are now tightening a decade after they ruptured over Iraq.

Washington has welcomed and provided logistical support for France's interventions to quell the spread of Islamic militants in Africa's Sahel region.

The allies are also key players in the group of world powers negotiating with Iran over its nuclear program, and blame President Bashar al-Assad for gross atrocities in Syria.

Hollande opened his three-day visit to the United States with a rare flight for a foreign leader aboard Air Force One on Monday.

The presidents then toured the Virginia homestead of Thomas Jefferson, seeking to forge a personal bond in richly symbolic surroundings.

The US leader said Jefferson, primary author of the Declaration of Independence, ambassador to France and the third US president, represented "what's best in America."

"What he also represents is the incredible bond and the incredible gifts that France gave to the United States, because he was a Francophile through and through."

Hollande added of the US and France: "We will remain friends forever."

Tuesday's Oval Office talks likely to focus on the war in Syria, Iran's nuclear program and Ukraine's political crisis.

The leaders will also likely discuss French military operations in Mali and the Central African Republic -- Washington has provided air transport, intelligence support and logistical help for both French missions in Africa.

A proposed Transatlantic Trade and Investment Partnership between the European Union and the United States will also be on the agenda, as Obama faces a tough domestic fight to win authority to negotiate trade deals in a mid-term election year.

On a personal note, Hollande hopes to leave embarrassment from his split from girlfriend Valerie Trierweiler -- France's former first lady, behind.

But her absence will be noted at the state dinner when guests will savor American caviar, quail eggs, rib-eye steak, and Vermont blue cheese, washed down with a selection of American wines.

Hollande, whose approval rating is the lowest ever recorded by a modern French leader, is under intense pressure to boost his country's ailing economy.

So he will try to generate some positive coverage for French innovation when he flies from Washington to California on Wednesday to meet tech leaders in Silicon Valley.