Trump, who has made few friends in Europe with his rejection of the 2015 Paris agreement and his "America First" trade stance, met with French President Emmanuel Macron as both leaders sought common ground to reset an awkward relationship.
"Something could happen with respect to the Paris accords, let's see what happens," Trump told a news conference.
"If it happens, that will be wonderful, and if it doesn't, that'll be OK too."
Trump has said the Paris accord is soft on leading polluters like China and India, putting US industry at risk.
"I respect the wish to preserve jobs, I think that's compatible with the Paris accord," Macron said at the joint conference.
"There is no sudden and unexpected change today, otherwise we would have announced it, but there is the shared intention to continue discussing these issues," the French president added.
Trump and Macron's relationship got off to a bumpy start, but both have an incentive to improve relations - Macron hopes to elevate France's role in global affairs, and Trump, seemingly isolated among world leaders, needs a friend overseas.
Trump came to France beset by allegations of Russian interference in the 2016 U.S. election, with emails released on Tuesday suggesting his eldest son welcomed an offer of Russian help against his father's Democratic rival Hillary Clinton.
Weeks after Macron hosted Russian President Vladimir Putin at the Palace of Versailles, Trump will bask in the trappings of the Bastille Day military parade on Friday and commemorations of the entry 100 years ago of U.S. troops into World War One.
Macron welcomed Trump with a warm handshake and smiles, a contrast to the clenched-jaw greeting they shared at their first encounter in May.
"Emmanuel, nice to see you. This is so beautiful," the US president told Macron as they met at the Hotel des Invalides where Napoleon Bonaparte and other French war heroes are buried.
Syria cooperation
For the 39-year-old Macron, France's youngest leader since Napoleon two centuries ago, the visit is a chance to use soft diplomacy to win Trump's confidence and set about influencing US foreign policy, which European leaders say lacks direction.
Macron views it as counter-productive to isolate the United States on the world stage, and said he and Trump had asked diplomats to draw up in the coming weeks a concrete initiative aimed at preparing the future of Syria.
"On the Iraq-Syria situation, we have agreed to continue working together, in particular on the building of a roadmap for the post-war period," Macron said.
Trump said work was underway to negotiate a ceasefire in a second region of Syria.
On Friday, Trump will be guest of honour at France's July 14 celebrations, a year after a Tunisian man loyal to Islamic State ploughed a truck through revellers on a seafront promenade in Nice, killing more than 80.
During the U.S. election campaign, Trump said a wave of militant attacks showed "France is no longer France", and reprimanded the then-Socialist government for allegedly bowing its head to jihadists.
In bringing Trump to Paris, Macron has stolen a march on Britain's embattled Prime Minister Theresa May.
London's offer of a state visit for Trump met fierce domestic criticism and warnings that he would be greeted by mass protests.
An Elabe poll showed that 59 percent of French people approved of Macron's decision to invite Trump.
