By John Irish

PARIS (Reuters) - The world must return to a policy of "maximum pressure" against Iran to turn it into a more democratic country, U.S. President-elect Donald Trump's incoming Ukraine envoy Keith Kellogg told an Iranian opposition event in Paris on Saturday. 

Trump has vowed to return to the policy he pursued in his previous term that sought to wreck Iran's economy to force the country to negotiate a deal on its nuclear programme, ballistic missile programme and regional activities.

"These pressures are not just kinetic, just not military force, but they must be economic and diplomatic as well", Retired Lieutenant-General Kellogg, who is set to serve as Trump's special envoy for Ukraine and Russia, told the audience at Paris-based Iranian opposition group National Council of Resistance of Iran (NCRI).

He said there was an opportunity "to change Iran for the better" but that this opportunity would not last forever.

"We must exploit the weakness we now see. The hope is there, so must too be the action."

Iran's foreign ministry, in a statement on Saturday, said, "The hosting of a terrorist group by France is a clear example of support for terrorism and a violation of the French government's international legal obligation to combat terrorism."

Kellogg has previously spoken at NCRI events, most recently in November, but his presence in Paris, even if in a personal capacity, suggests the group has the ear of the new U.S. administration.

He postponed a trip to European capitals earlier this month until after Trump's inauguration on Jan. 20.

It was unclear whether he would use his trip to Paris to meet French officials to discuss Ukraine. The French presidency, foreign ministry and Trump's transition team did not immediately respond for comment. 

Incoming U.S. Secretary of State Marco Rubio has also spoken at NCRI events in the past. The group has repeatedly called for the fall of the existing Iranian authorities, although it is unclear how much support it has within Iran.

Speaking at the start of the event at Auvers-sur-Oise, the group's headquarters on the outskirts of Paris, NCRI President-elect Maryam Rajavi said the regional balance of power had shifted against Iran's leadership with the fall of Syria's President Bashar al-Assad and the "crushing blow" suffered by its most important ally Hezbollah is its war with Israel.

"It is time for Western governments to abandon past policies and stand with the Iranian people this time," she said.

The NCRI, the political arm of the People's Mujahideen Organisation of Iran (PMOI), has held frequent rallies in the France, often attended by high profile former U.S., European and Arab officials critical of the Islamic Republic.

(Reporting by John Irish; Editing by Alex Richardson and Christopher Cushing)